Yep, i'm an excessive worrier too! Just today at my counsellors session, i went on a big rant(all the while becoming more and more anxious!) about the fact i've been invited to two 21st birthday parties in the span of a week, which means i'll be hit on by some guys, and god forbid i actually like one of them, that would mean a possible relationships, and all my relationships have always turned to crap! Haha, so yes, i think that about sums it up!
Cactus jack has it right though, when you think of one situation, it almost automatically leads to another and so on...
I can't be of much help in ways to deal with it, i'm just starting to tackle it in therapy myself!