Recently I have started Yoga, but I find myself unable to hold any position more than ten seconds. I was told to exaggerate my posture, but I simply could not. My flexibility is horrific and feel counter productive even when I follow the Yoga books and routines (books and videos). It has been two weeks, and I am having hard time improving it. Is there a modified way to help me get into the basic positions of Yoga before I can at least be effective in beginner Yoga workout? How can I improve?
Two weeks isn't adequate time to make a determination if you're not in good shape to start. You will improve if you are following the directions properly. Just don't give up, and stick with it.
From your question, it sounds like you're probably also doing this at home via multiple sources, media, or routines. Pick ONE, and stick with that one. Don't try to follow multiple routines or programs, find a single routine, and practice that single routine, nothing more or less. You will see much more improvement in that manner than you will by trying to work from multiple sources simultaneously.
These things take time, don't worry about it. If you are consistent you will see results, but plan to evaluate at least every few months, not weeks.
If you have difficulty maintaining your motivation, join a class, that really helps a lot. You don't want to give up on holding that position when the person next to you is still doing it (even though they are probably thinking the same about you). Also, even if you fail to hold it for the given time, don't just give up. Let's say you have to do 30 seconds and collapse after 10. No problem, relax for a few seconds and then go at it again. You will fail in five. Relax, try again. Keep pushing yourself until the proper time expires.
Of course this goes for resistance exercises. Don't try to push yourself on stretches too much, that's a guaranteed way to injure yourself. You should feel tolerable discomfort, not agonizing pain.
But really...the class will help. Also some exercises seem simple, but there's a subtle catch to them where the instructor can help you a lot.