Clothears, I don't know if I can help at all to set your mind at rest about the relationship you have with your DD being affected - I'll try.
My DS1 was 21mo when I had DS2, so he was exactly a year when I fell pg. I never worried that I'd be able to love another baby enough (you do, of course you do - love grows big enough for everybody, and the more there are of you, the more there is to go round somehow), but I did worry about DS1 and how he'd be affected by having another sibling while he still seemed so little and dependent on me.
He's a sensitive little boy (he woke in the night the other night, crying because Thomas Tank Engine had "too many jobs to do" ), and he worries about things (he's 2.9 now).
He was fine with DS2. Utterly, totally fine. They adore each other. In fact, having DS2 around has been very, very good for him, and for me - I've not had the time to worry as much about the little things with him, and I think he's benefited from that, because I have a tendency to over-think things (never, I hear you say!).
DS2 is a boisterous little chap - very different from DS1, but they complement one another very well. DS1's job is to remember DS2's Panda when we go out - and he often says things like, "Don't worry, DS2, Grandad will fix it/Daddy will wipe it/Mummy will help" - he's always the first one to tell me when DS2 needs milk, or if he's crying(!).
I am pro-choice, very definitely, so this isn't intended to be a guilt-trippy post. I can't tell you what to do, I don't know the ins and outs of your mental health or your relationship with your new partner (who, from what you've said he sounds like a decent chap). I just wanted you to know what it's like on "the other side", to hear it from someone who worried about having a small gap between children, who worried about the relationship with her firstborn - but who needn't have done.
I wish you strength and happiness in the future. Keep posting, keep talking to people (on here and in RL), and don't make any hasty decisions x x x