Have you tried meditation, such as the mindfulness app? I can't say from personal experience but i know for other people it has helped.
I have always suffered from anxiety and excessive worry but found it only affected me mildly now I'm pregnant. I got myself an early private scan at 9 weeks which reassured me a lot.
The majority of pregnancies are fine. The majority will have no or minor complications. Just remember, what you read in mumsnet probably is not representative.
People come here perhaps due to past negative experiences. Or like you and me, due to excessive worry.
You can read through posts here and they can make you feel more worried because suddenly mc seems like a normality. But it isn't. Yes during early pregnancy there is more "risk" but the risk of mc is still smaller than the likelihood that everything will be fine.
I find when my brain turns to negative thinking (i have more social anxiety) I have to train myself to think the opposite. I have to tell myself these feelings are exaggerated, they are unreasonable. That I don't know that for sure, that I'm projecting my own negative thinking into how i think others think of me. It's very tiring being anxious all the time.
You need to be, i think, open about it. Say, I'm feeling anxious a lot. Talk to your mw.
I would recommend an early scan if you can afford it cos 12 weeks is just too long to wait. It cost me £90 so yeah, not cheap but not totally expensive either and worth it. I saw quite a developed baby at my 9 weeks. (They dated me at 10).
It might give you peace of mind, but be sure not to become addicted to them. I gotta say i am so excited to see my baby again next week at my 12 week scan. Having to wait until 20 weeks to see it again seems too long!
So in summary. Concentrate on the positives. Write them down if you have to. Allow yourself space to worry a bit, i don't believe trying to banish negative thoughts forcefully helps. But introducing more positive things. Like what you are looking forward to in hope that they will replace your negativity in a more balanced fashion.