Lea, I just want to say that everything you're saying rings very true for us too. In fact I've found myself at multiple points in this pregnancy questioning whether we've done the right thing, which feels terrible given how lucky I know we are. We ummed and ahed before trying and then gave ourselves a very limited window in which to try before stopping and accepting the family we already were. It was only when we got that BFP that it dawned on me that I really hadn't expected it to work given our previous history, so this pregnancy has seen quite a lot of evolution in my thoughts and feelings. Of course, the primary feeling is one of great happiness, and I know that as soon as we meet this baby we're going to fall head over heels in love again and won't be able to imagine a world in which they weren't a part of our family too. But right now, even so close to them being born, I still find myself questioning and even grieving for the one-on-one time I'm going to lose with Ember (quite ignoring the fact that she goes up to four days a week at play group next week).
I've found it very useful to talk to others who are planning or expecting a second child - through that I know these feelings are extremely common so I don't feel so bad about having them. I also know that everyone says that it stops almost immediately once the new baby arrives, probably because there isn't time to dwell on anything in the chaos that ensues but also because that unconditional love kicks in again.
Of course, this is all very easy for me to say. We're possibly just days away from being a two child family, but after going through three and a half years of trying last time, two donors, three miscarriages, and more heartache than I thought we could bear I do understand at least some of what you're going through. There's a meditation/hypnotherapy CD that I used the first time which I found really helpful in keeping my stress and anxiety levels down and which made a massive difference to how positive I felt about the whole process. It was the 'prepare to conceive' CD (not a 'trying' in sight) by Natal Hypnotherapy: https://www.natalhypnotherapy.co.uk/5.html. I'm not sure if that kind of thing would be up your street or not but it was very helpful to me.
Sending big hugs all round (they're very round ones coming from me right now)
Gina. x