I hate feeling this way....?

Everytime my boyfriend and I bicker about something the first thoughts that come into my mind are negative. I start invisioning these scenario's where he's acting like a complete a**hole or that he cheats on me, etc. horrible things! Although he's never given me any reason to think that way, I can't help it. But the problem is that it just fuels my anger about whatever we're bickering about and makes it worse for me to deal with it. I hate being this way, I don't know why i'm acting like this and if I were a guy I'd be upset with me. LOL! Somebody please smack some sense into me? Why does this happen?