I have a similar experience. I started talking to this girl who went to my highschool, we didn't talk much during it but when we finished, a friend if mine decided to make a discord server, added them and we started talking alot more. We then decided to make a new serve with one of my friends and one of hers and this is where we would stay up till 4 almost every night messaging and talking ab anime and games and just random stuff. Ofc I started to develop feelings, however I never told her ab it. I waited 5 months before I felt ready to tell her and just before I did, I messaged her friend, who already knew I liked her, to tell her I was going to confess I liked her. She then informed that she had a boyfriend already. For 2 weeks I didn't say anything and found myself, getting really frustrated and angry at little things and thinking in the "nice guy" mentality. So I ultimately told her that I liked her but made it clear that I knew she was in a relationship and that I wasn't trying to get in between her and him, I just needed it to be out in the open and off my chest. She was really understanding and we're still friends and it's not awkward at all.

I think you should just tell her.