I have to say that, given the context, I'd vote (as though I had one!) against ex-anything: everything about the chapter is soft, and ex- would add a shockingly harsh note, to me. I think that with the explanation, beloved works--er, Beloved. Yes, it does have to have a capital! Beloved emphasizes how strong the love was. Though I still call mine my First Love; it was most certainly unrequited--I was 13, he 28. It never flagged, as I saw him (not dating--was physically in his presence) occasionally over the next several years. I still get reminiscently starry-eyed when I think of him.