I think "oh God what have I done?" is a pretty normal thing to think. There are very few life decisions you can make that have as big an impact as whether/when/who with to have children, if that makes sense.
At this point, the question is no longer "Should I get pregnant?" but "Would I rather keep the baby/have an abortion/put baby up for adoption?" which is a bit different. If you're leaning towards option 2 or 3 then it may be worth asking your GP to refer you for counseling re: pregnancy choices.
On the other hand, if you think the issue is more anxiety about the future - What were the reasons you wanted a baby? You don't have to list them here, but it might be helpful to think back over them. Have any of those things actually changed?
A perinatal psychologist told me that 70% of new mothers report feelings of ambivalence towards their baby in the early days. It is normal, and not a sign of impending disaster.
What is it about your life as a mother that you might hate? It might be helpful to give some examples - and then think about the likelihood of it happening, how long it would happen for (children change all the time), how you could manage it if it did happen, etc.
Part of what makes pregnancy hard is that there are so many social rules about what mothers-to-be are meant to think and feel. We're supposed to be on a pink fluffy cloud and thrilled by everything and want to be totally absorbed in baby stuff but it's OK to not feel like that. During pregnancy a woman's body and lifestyle and world view and identity change faster than at any other point in your life, all at the same time, in ways that are really hard to predict.
What Nobody Tells You by Alexandra Sacks is a book that goes through some of the mental side of pregnancy, including practical advice on how to deal with issues that lots of people find difficult. It's worth a look.
Most people have heard of Postnatal Depression. Antenatal anxiety is also a thing, affecting around 20-25% of women. Left untreated it can make your pregnancy b thoroughly miserable and make PND much more likely. Talk to your midwife (when you meet her) about how you're feeling. If you feel fobbed off, or if you're not going to meet her for a while, go and see your GP. There is help available. (There's also good info online, MIND and Tommy's are good places to start)