Its not rocket science. The fact that he doesnt want to have sex with you has nothing to do with stress. When he told you it wasnt you, he was being only partially honest. Sometimes guys just get tired of having sex with the same girl for so long. They don't mean to hurt you in not wanting to have sex anymore. It's just become monotonous to them. Its like if someone were to ask what your favorite food was. Lets say it was steak. Would you still like steak if it all you had to eat for 3 years? I know I wouldnt. Other foods would start to look pretty enticing to me while steak would almost seem revolting. Let me ask you this, though. Have you gained a little weight since you've started dating him? If you have, this could be another reason he isnt as sexually attracted to you anymore.
Anyway. Guys arent that hard to figure out. There's a reason he prefers pronography to sex now.