I do feel really sorry for you lisa, I had similar anxieties in my first pg, dp and I were in a long distance relationship, and I was young and a bit of twat when drunk (there were a couple of very drunken nights, it was xmas/ny).
I convinced myself that the HIV test they give you would come back +, to the point of almost hyperventilating before midwife appointments, I also convinced myself that I had shagged a randomer when drunk and my baby would come out a different colour 
.
Despite knowing (with hindsight) that I had felt that familiar twang of ovulation pain the same day that dp and I went to a gig, got roaringly drunk and forgot the condom.
Good for you (if this is real) for verbalising your worries, I did the whole torturing myself in private thing.
All turned out fine btw, I didn't have HIV or hepatitis or whatever, dd came out looking well, like dd really. She does look a lot like my subsequent children though, so I'm sure dp is the dad.
Hopefully you get the help you need to control your anxieties, a worry loop is not a fun place to be, it can also easily lead to (or be a symptom of and/pnd).
Keep talking, you are much braver than me.
Good luck, I'm sure you can get through this and really enjoy your little one 
.