Here's a modified hamstring stretch that might be easier for you to start with: www.sparkpeople.com/resource/exercises.asp
The good thing about flexibility training is that if you do it consistently, you can start to see results pretty quickly. Make sure you always warm up before stretching, but if you can start doing hamstring stretches frequently, you should notice they will start to loosen up soon.
"You may have a fresh start any moment you choose, for this thing that we call "failure" is not the falling down but the staying down." Mary Pickford