Distress of Mind and Body

Wow, I can't believe what I did yesterday and sabotage prolly one of the best things in my life! Well it was more than one thing, but in any case, I still did it. Why I don't know. Let me elaborate into this more...

Yesterday I was at work and I was joking around with my roommate and giving him some shit that he had spend most the night up with my Boyfriend. They were up till about 4am. I had gone to bed about midnight or so. I texted John and gave him shit that they stayed up for so long and they must have had some intense "conversations" right after that is were I made the mistake of opening my big Sagittarius mouth. I told him that I was upset somewhat because the entire time, in my weird and tired mind state, that they were only making out or getting it on. Yeah that was not a good thing to say or think. I do trust John and I know that he wouldn't have done that. For some reason though my mind was thinking the worst possible things like that. My mind set is my biggest relationship killer. I always assume the worst possible thing, then I start having myself believe myself!
I feel so bad and aweful for even saying anything. What is even worse is what I did to my Boy last night.
We both thought to try to get to bed early last night, since he had to be up really early for meetings at his work and what not.
Well everything was going great and we kissed and yada yada yada. Well we were getting to the point of actually having sex and I froze. Its like I had someone put my mind and body into a freezer. We were both really hot an ready to go then this happened. I told him that I think I might just go ahead and go to sleep. Well he snapped at me and I believe he was really pissed at me so I didn't say anything. I am not very good in arguments and I had already felt so bad for "teasing" him like that then just putting a stop to it.
Its like I am scared to actually have sex or something. This isn't the first time this has happened to me and my partner. In fact its been the downfall of almost all my relationships to date. I want to be able to get to the root and figure out what to do. I just don't know where to start. I have tried to talk with a few friends about this some time ago, an they just tell me its in my mind and I think to much. Well yes that is part of the problem, but what makes my mind kick into that gear that is detrimental to me. I just don't know