It is pretty normal not to feel much about an abortion, just like it's pretty normal to struggle with the decision, or to grieve afterwards (even if you are sure it's the right choice and don't regret it). Abortions are a very common medical procedure (according to Guttmacher, 3 out of every 10 women in the US will have one by the time they're 45; a study I saw put the number at 1 in 3 for women in the UK), so there are going to be a wide range of reactions to it.

I had one when I was about your age, and I also worried about the fact that I wasn't that upset by it. I never doubted that it was the right decision for me, and afterwards, I mostly felt relief. It's been quite awhile since then, and I only really think about it when threads like yours come up. When I was waiting for my appointment, I was able to look through a book where a lot of women had anonymously recorded their experiences (this was long enough ago that turning to the internet wasn't really possible). A lot of them talked about being sad, or scared, or conflicted, but a lot of them also felt like I did.

As far as your fiance goes, this is one of those things you just have to trust someone on. I know it's not easy when you have anxiety, but you have to accept that the possibility of him maybe growing to resent you is not something you can control. He is saying that it is okay with him and he wants you to make the best decision for you, so take him at his word and do this. If he changes his mind and starts to blame or resent you, then maybe he wasn't the person you think he is.

I do think therapy may be a good idea to help you get a handle on your anxiety and depression, if you are not currently seeking it, especially because your hormones are likely about to go crazy again and that may further affect your mood.