I felt exactly how Fork describes - they "oh my god I've just voluntarily given myself a life sentence" feeling. It can be very very hard, especially if you find that however delightful their newborn sqishiness is, babies don't do a whole lot beyond making demands. I would imagine those feelings could well hit during pregnancy.
Your life does change out of all recognition, and there will be stuff you used to do that you can't any more (for me, dangerous sports).
But I've found the plus points outweigh the minuses, and also as he's got older I've got a lot of myself back. Also, I now do adventurous things with him (still remember the look on the other climbers faces when I popped into the summit of a rocky peak in the Alps with my 7 yo on the other end of the rope).
One thing worth bearing in mind is you can have ante natal depression (I had one brief episode of this) so if these feelings have suddenly come out of nowhere maybe that's a possibility worth considering.
I used to joke when DS was a baby that I wanted a t-shirt that said "this is not PND, this is a rational reaction to intolerable circumstances". In retrospect, I'd say I did have PND, but also that there was a lot of truth in my imaginary t-shirt - those first few months are very very hard.
But it gets easier and I can now honestly say he's the best thing in my life (still not necessarily the easiest - there are times I think "boarding school, why didn't I send him"
).