Well, I think the first step is to be brutally honest with yourself for a minute. There has to be some hidden (or maybe not so hidden) reason for the freeze up at this time.
Could it be that you feel your performance in bed isn't good enough? Might be hard to admit, but that could be something holding you back.
If you don't mind sharing, how often do you have sex? When was the last time? How was it?
EDIT: I was looking in New Posts, and didn't realize this was in the High School forum. May need to switch up advice here, but feel free to answer my previous questions first, if they apply.