Obviously you're just not ready. Maybe the difference between this time and the first time you were with a woman is because the reality hadn't sunk in. You're obviously grieving your old relationship and that is usually accompanied by some depression. I have a male friend who went through the same deal.
If you think you like this person why not just be honest and say that you're not ready. Women, I think, can take this explanation a little better than men. Just let her know that it's not about her.
You'll be fine. You just need some time. Have you thought about talking to anyone?
