You could go to relate or mind to talk about this.
I think you need to speak to someone ( a counseller) to help you unravel this big ball of black cloud you've knitted yourself. That would be my step one
Step two. Following the counselling, you then can have a clearer path to a solution. It's going round and round in your head op like a cat chasing its tail.
to get a bit of a male perspective I asked OH who is a very sensible and level headed guy and like me he feels that it's best to take a step back and look at the bigger picture.
Deal with the present. There's a baby on the way and everyone is happy about it( presuming from your posts)
So, you need to keep yourself well and healthy .
There's a strong probability this baby is his.
How many tests are wrong. How many people without cancer have had bits cut off, chemotherapy etc on the basis of a wrong test result. How many people have been wrongly diagnosed ( my cousins aggressive cance was passed off as piles for three years) thanks to a misdiagnosis. So you need to get some perspective on this.
You feel guilty that's true but against the big picture it's about living with your guilt when the baby is highly likely to be his and you could settle into domestic bliss with no need to worry about the baby being I'll, needing genetic testing etc
I'm sure my cousin is bringing up someone else's baby as they had split up and she was seek g someone else but the baby is loved dearly by everyone and the couple are happy so who's to argue.
Don't worry about the future. You are torturing yourself with things that may not happen. Deal with the present.
But yes, have a chat/ counselling sessions with the professionals.