During exercise I often notice that I'm physically capable, but not mentally. At some point a strong mental listlessness makes me stop my workout.
Normally at this stage a training buddy or a personal trainer would give you a good kick in the butt to create the motivation to do that extra repetion or that extra minute on the treadmill.
How can you do this if you are on your own? How can you build up enough willpower to push you through in the moments where your willpower is at its lowest?
Here's what works for me
Music
If you don't have a taste for techno, develop one. The high BPM (135+) songs not only give you a beat to match; they also give you a rate of momentum to match so you can measure when you're starting to fall off.
Try to sync into your own little world. One of the greatest barriers (at least for me) of working out is the social awkwardness of doing it alone. The better you're able to tune out the people around you the less anxiety you'll feel about doing activities by yourself.
I use two different (and both very high quality) podcasts for workout music. Podrunner and Beatport Burners.
Learn how to make snap decisions
It may seem pretty rudimentary but the ability to make very quick precise decisions is a good skill to have when you need to step out of your comfort zone.
Here's one strategy I've used to build this skill in the past. When you wake up in the morning (for work or school) instead of milling around in bed, immediately jump out of bed as if your house is on fire.
The more time you spend dwelling on a decision the harder it is to make.
Another would be to literally split the decision into two different options and flip a coin to decide. Instead of a lazy/active decision make it an active/active decision. For instance, instead of deciding whether or not to go workout, make it 'workout at the gym' or 'go for a run'. That way you have no path to laziness. The coin is only used as a crutch to get over the anxiety barrier involved in decision making.
IMHO, contrary to popular belief, indecisiveness has very little to do with laziness and very much to do with anxiety. Many people are lazy simply because they're too afraid to break out of their comfort zone. The reward (benefit of getting out) has to be greater than the risk (fear/anxiety involved in the unknown of trying something new).
Shaming yourself into believing that your just a 'lazy person' only makes it harder to become motivated because now you have to overcome both the anxiety of trying something new and the insecurity created by your self-image as a 'lazy person' (ie the risk increases).
Imagine the different ways you can critique a person. First you can attack a person's actions (why don't you get off your butt and get outside), attack their traits (why are you being lazy), or attack their person (you are a lazy person). The first two are easy to change, the third... not so much because you're describing the essence of what that person represents, not their actions. Take this idea into consideration when you critique yourself. People have the singular ability to destroy their own self-esteem without any external interaction because of this. Learning how to separate the 'who I am' from the 'what I do sometimes' is an important skill to learn to prevent from any unnecessary confidence speed bumps.
Adding an imaginary sense of danger is a means to increase the reward of getting out (escape from danger) whereas flipping a coin leaves the decision up to a higher power (luck, god, ect). I could probably write a dissertation about how religion relates to the higher power aspect of decision making but I'll spare you.
Diet/Health
A mental barrier is not always strictly mental. Sometimes, a lack of energy/motivation can be due to your diet/health. For instance: if you're not getting enough sleep your hormones are likely not balanced properly; if you overtrain, your body hasn't had enough time to properly recover yet; if you don't have enough energy you will feel like you don't have enough energy.
The solutions to these issues are simple: get enough rest; plan enough recovery time in between workouts (if you start to feel a decrease of energy); and eat enough (carb load the night/morning before a day of high energy activity) and be sure to eat a lot of protein right after an activity (protein uptake is greatest <45min after exercise) for a better recovery.
I have dealt with and continue to deal with these issues on a daily basis. I could probably be considered a high anxiety personality type (even though my general demeanor is extremely relaxed) because it takes an extra conscious push for me to get involved in a lot of the activities I do.
Fortunately, I've found that the more I get involved in, the easier it is to see the reward instead of the risk when it comes time to make those snap decisions.
Update: @Ivo Flipse just recently posted How Can We Get Willpower Back Once It Has Been Depleted. It's a good read and definitely related to the Op's question.
One of the things I like about the 5x5 style lifting programs is the following model:
On one of my lifts, I have the same mental issues. I have this lingering doubt in my mind that this is the session I'm going to stall on. Yet, I push out the reps and keep going. It's that process of overcoming the doubt 5lb at a time that helps me keep pushing. I want to see how far I can go without stalling. I know how many reps I need to do, so I do it.
Now, when you are lifting on your own, you simply can't push yourself as much as when you have someone there to push you. Part of the problem is that when you have someone else as a spotter you have a safety you don't have on your own.
My suggestion would be this:
It's easier to convince yourself to push out a rep when you know it's the last one. At some point you have to tell yourself to shut up and work. Every time you push past the last limit it builds your confidence that you'll be able to repeat history.
I agree with the other answerers about music, planning, and gradual improvement.
When it comes to gradual improvement, I would also like to emphasize that tracking can also be a huge motivator. What's your personal record for this exercise? Can you try to pass it? If you ran 1 mile last time, can you run 1.05 miles this time? If you lifted 100 pounds can you lift 102.5? If you finished your circuit in 10 minutes can you do it this time in 9:50? If you are determined to at least improve on your last workout you may find it easier to keep going.
To help my track my progress I graph all my workouts using Microsoft Excel. It's fairly easy to record your numerical records in a spreadsheet and create a graph out of it. When you look back on the last 3+ weeks of records and see the lines on the graph moving steadily upwards, it's very motivating. You want to work out because you can easily see what you're getting out of it.
The other thing I've used before when running is always giving myself 1 more minute before I quit. For example, I may have planned to run for 45 minutes. It's now been 30, and I'm really feeling like quitting. My muscles ache and my feet hurt, and I'm tired. But I don't feel like I have to quit, I just want to. I think to myself "I could quit now and still be happy with myself. I don't need to run the last 15 minutes. 30 minutes of running isn't bad. I can do better later... BUT, before I quit let me see if I can run just one more minute." When that minute is up, I think "Ok, it's been a minute, I can quit now. But... could I go just one more minute before I do?". Ocassionally I really do quit, but often I find that I'll give myself "one more minute" all the way to the end of my planned workout, and I never do quit. It's that mental trick of giving myself an out, then thinking about just slightly delaying it, rather than focusing on the enormity of what's left, that allows me to keep going.
Like George Throrogood says - I drink alone when I'm by myself.
Working out alone isn't for everyone, if you feel lack of inspiration or mental fortitude when you're by yourself, you have two options: change your situation and find a workout partner (or trainer) or learn to inspire yourself.
How can you inspire yourself?
You may end up not being able to workout alone, many people can't, we (most of us) are social animals and need that interaction. If the issue with a gym or personal trainer is money related...post on workout forums for local partners.
Well, maybe this is not really the answer you are looking for, but some people might find it helpful.
There is an interesting website for forming and finding groups:
www.meetup.com
Here where I live in the Netherlands there are a couple of running groups that you can join to start running. That could help with motivation.
Also you could start your own Meetup (I've started my own for people learning to program). I have thought about starting a meetup for freelancers to go to the gym together.
In your case I am imagining something like this: A gym-buddy meetup, where you just go sit somewhere like once a month, get to know other people who are going to the gym or who want to go to them gym. And than that could lead to people arranging to go to them gym together.
I bet there would be a lot of people around the world who would like an initiative like this, but who don't want to start one themselves, for whatever reason.
And of course, this can be organized any way you see fit (no pun intended).
O yeah, and I notice that taking a cup of coffee before training helps me with motivation. And some nice upsdrum and bass music.
Also, short training sessions could help (like 30 - 60 mins). I don't like to do a lot of cardio machines, but I would recommend doing a high intensity strength training workout. Finding a good trainer for the first few sessions might be a good idea. You need to at least get some good advice on which exercises you can do, and how to perform them correctly.
