Thank you all for the very helpful perspectives on the matter.
To answer a few questions that were raised -
1) I believe I am mono because while I was with the married man previously he and his wife both actively encouraged me to date other people and "find someone else to fall in love with," and I did try, and got interested in a few other people but didn't fall in love. The same has happened in the intervening months with Aarin - I've met other people but can't seem to change my "love focus" for lack of a better term.
2) I think the idea of talking to Anika and Aarin and seeing if I can find a way to be Primary while also giving Anika the place in the relationship she expects and deserves is a good one.
3) As to what I want, which has been raised by a few people, I know for sure that I want to be with someone long term right now. I don't want to keep dating, I definitely want to settle down with someone. Marriage, kids, I don't know, but certainly stability and a committed partner. I also want someone with whom I don't have to negotiate time or attention. I've been feeling while Anika has been processing a lot of things that Aarin's attention is really divided and I can't ask for the time I want with him when Anika's hurting. I'm not sure how to navigate that when she's working through stuff, so I've left it be.