I can sympathise - I have OCD and am 6 weeks pregnant. Part of me is soooo excited and wanting to tell the world (though are waiting until my 12 weeks scan!) though part of me is obsessing about what if something goes wrong / is wrong with the baby!! Am driving myself nuts!!
However my rational self knows that everything is alright and will be alright.
I'm healthy and sensible, have no illnesses, there are no 'conditions' in my family, etc etc so the odds are very much in our favour and there is no reason anything will go wrong.
It sounds like you too may have some form of OCD so seeing a psychologist / therapist should help. I've had CBT and highly recommend it. Even though Im still struggling sometimes, I am nowhere near as bad as I have been in the past. I literally used to spend the majority of the day winding myself up, obsessing then doing silly rituals / compulsions to counteract them - I was a little crazy!!
With me, I know its the OCD that is the problem not the thoughts. If I wasnt obsessing about my baby it would just be something else. I've had OCD for 10 years and have obsessed about all sorts of things from ex-partners to conversations at work to knickers (dont ask!!). The thoughts are irrational and irrelevant; the OCD is a big bully but it can sod off!! You dont need it, esp. when you're pregnant.
Other advice includes keeping busy (read, watch a film, play a game, listen to music, see friends, come on Mumsnet!, etc) , eating healthily, making sure you look nice (look good, feel good!), relax when you can.
All perhaps easier said than done I know - and you cant switch your brain off! - but it comes with practice.
You dont want to regret not enjoying your pregnancy - its such a special time and wont happen often!
Anyway, Im here if you need to chat more - I can understand (and may need a little helping hand myself!). I find Mumsnet can be a good support.
Be nice to yourself and your soon to be beautiful baby!! 