Ok you do need to think carefully. I only say the following as it's quite clear that a big part of you doesn't want to terminate. And to have a termination that you don't want is something you need to take some time to think about.
For all the reasons others have listed above, terminating is a sensible and logical option - most of all, so that you wouldn't be tied for 18+ years to a man who has shown himself to be thoroughly unsuitable. Even if you didn't stay together, you'd have to deal with him on some front... and remember, that could include him becoming enthusiastic and wanting very much to be in your child's life, possibly not in an entirely positive way.
BUT.
You know you want more children. You have also said that you don't want a huge age gap. There's also the potential worry about gynae issues and conception.
If you terminate, you are pretty much definitely going to have a big age gap. If you don't end up with at least a ten year age gap (ie give it another three years at least) until you conceive again, then it's a given that you will have got pregnant to another man that you will by definition only have known for a couple of years at most. I'm not sure that's a better position to be in tbh.
There is also always the possibility that you don't meet someone decent easily. Then, you'll have that looking over your shoulder panic - I MUST find someone, I'm getting older, I want another baby. Meaning that as time goes on, you're more likely to settle, or choose poorly, because the wanting a baby and a sibling for your DD will get more and more important.
If you kept this baby, yes, you would be a single mum again (not what you wanted) and this particular idiot would indeed be the dad, in or out. However - you would have had your second baby, and (presumably) completed your family (or at least had a second baby) with the minimum age gap you're ever going to get, now. You're so young that that would mean that the pressure would be completely off - and, thinking in terms of choosing men wisely for your whole life in the future - that's not a bad place to be. It would mean that you would go into your late 20s thinking - if the right one comes along, great. If he doesn't, I don't have to panic about meeting him and pushing the relationship to baby stage asap. That's a strong position for you to be in as the mum of two children.
Also - you'd be so young having finished with the baby stage that, although as you say you'd initially really have to rely on family, by the time you were 32, you'd have them both in school and could be flying career-wise: again, no pressured thinking on how you will end up going right back to square 1 at some point because you know you want a 2nd baby. So that initial difficulty might more than balance itself out by ending up in the rather nice position of having got all the expensive baby stage out of the way before you even hit your mid 30s.
But by far the most positive effect of taking this raod, to me, would be the fact that you would not spend the next few years a complete hostage to unsuitable men because you know you want another baby and time is marching on. To be freed from that requirement is a very, very good place to be.
So. You COULD choose to look at it that way. You could also very sensibly terminate, concentrate on your DD and accept that yes you will have a bigger gap, maybe just one child. That too isn't a disaster.
Good luck.