For the last four months I have been doing the Insanity workout. During this time I have decreased the amount of food I eat to about 1800-2000 calories a day. I drink the recommended amount of water one needs (about 8 cups) daily, and I never eat junk food. I drink a whey isolate protein after my workout (one 29g scoop), I take the ACG^3 pre-workout formula before exercising (one scoop), I take 12 Censor (a CLA supplement) capsules daily, and I get about 8-8 1/2 hours of sleep every night. Over these four months I went from 195lb to 175lb. I'm much stronger and feel really healthy, and I'm fairly tone everywhere except for my stomach. I exercise my abs daily and they've gotten lots stronger, but I'm not seeing definition unless I grab my excess flab and pull it down so I can see where it is covering up my abs. What's going on? I want the definition, and I'm working really hard every day for it. The abs are definitely there, but how do I get it to look like they are there?
I would suggest keep doing what you're doing. You lost 20 pounds in 17 weeks. That's great progress. That progress will necessarily slow down, though.
Ab visibility is largely related to body fat percentage. My guess is that you're somewhere around 20% body fat (http://www.builtlean.com/wp-content/uploads/2012/09/body-fat-percentage-men.jpg). (I'm assuming you are male from your username.)
Since you're still losing weight on your current plan, it's working for you, it will just take more time.
Lasting longer through ab workouts isn't training ab strength, but ab endurance. If you want to focus on ab strength, you need to progressively load them, and that is perhaps best done by working them isometrically as part of a larger full-body exercise, like the squat (http://startingstrength.com/articles/abs_rippetoe.pdf).
I don't know what your body fat percentage is but you need to have around 10% to have visible abs, mine is lower and I still barely see them.
One way to make your abs more visible is to do actual weighted ab exercises. Most fit people can do well over 20 situps which is no longer stimulating hypertrophy, if you try situps with some weight you may see better results.
I have to go back to my original point which is that body fat is mostly what matters, sometimes people who are not even in good shape have six packs solely because they are skinny, you are clearly doing a good job with your workout (lost 20 pounds) I say keep going as you are but increase the intensity of your workout that is the best way to get results, always keep trying harder.
I thought of one thing you can do but note that this is not necessarily healthy. Once you are around the BFP I specified you can start cutting water weight to make your abs show more. I suggest google searching this or ask another question, I'm not a body builder and don't have any reason to cut water weight so I'm not that knowledgable about it.