Lesbian friend zone— sort of?

I recently dated this girl who I really clicked with. We had a lot in common and had a great emotional connection. It’s hard for me to connect with people on that level, so I was really happy. However, she was dating another girl at the same time as me, as we weren’t exclusive. She was very honest with me and said that she is a serial monogamist, which I respect. She said that she had already slept with the other girl and had established a chemical bond with her, but was still having a hard time choosing between the both of us. She ended up choosing the other girl, which I expected would happen. However, she said that she still really likes me and finds me very attractive (and enjoys kissing me). She also said that it was all due to timing, and that she may have chosen me over the other girl had we slept together first. She said that she still wants us to be friends, and that she hopes that fate may push us together if her and the other girl don’t work out.

I really want to stay friends with her because she’s amazing, but I know I’m just going to end up disappointed and hurt. Our entire friendship I would be hoping she and the other girl would break up, and I would never be able to move on. I just don’t want to be the backburner, because that’s what always seems to happen to me. However, the thought of losing her is really painful. I still really want to be with her. But every time I go to tell her that I can’t just be friends with her, I chicken out because I’m afraid of losing her for good. She has already asked me to hang out, which I declined because I already had plans. But I know she will ask me to hang out again soon. If anyone has any advice on what I should do, I’d be eternally grateful.