Didn't want to read and run when you are feeling so isolated. I'm just over 11w with what will hopefully be my first child (last two pg ended in m/c). I can't relate completely to what you're saying - my fear is mixed with excitement and I can talk openly with my husband and close friends about all of it. That said, I am finding pg an isolating experience in some ways.
For what it's worth, I think it's completely natural to be scared and have doubts. This process involves our whole lives as well as our bodies changing in unpredictable and uncontrollable ways. Almost every parent friend I have talked to about my worries and doubts says they felt somewhat the same - but most felt they couldn't express it openly. And I've read a lot about women who were extremely ambivalent about their pregnancies the whole way through.
The bright side is that very few parents I've talked to or read about regretted their choice in the long run. For me, it helps to think of the whole process as a massive leap of faith - and to not beat myself up about any negative feelings I have about it.
Back to you:
Are you scared about pg itself, the birth, parenting, or the lifelong commitment? Are you scared of losing your own identity in becoming 'a mum', or not being able to have aspects of your old life back once you have a child? I've felt aspects of all of this at times.
Does your partner know all of how you're feeling, or do you feel you have to sanitise what you share with him because of his own excitement about your pg? Are you completely honest with the perinatal therapist about how much you feel you may have made a mistake?
The fact that you feel badly about your feelings, and that you're so lonely sounding, suggests to me you might be keeping a lot to yourself out of a sense of shame at not being 100% delighted. If you can talk honestly to some people about how you're feeling, especially those nearest to you, I think that would help a lot.