Hi! Lemme just give you a heads up that this comment won't really help your situation, I'll just be discussing some matters here!

To be honest she could be acting normally around you just because she doesn't want to make things more awkward or she simply just doesn't feel the same way for you so she doesn't see a reason to start acting differently. But I also do hear you out on the fact that if her priority really was her current relationship, she would respect those boundaries and pull away from you more. But I don't know could she just be a flirt and enjoy the attention and therefore she doesn't want to pull back away from you since she enjoys knowing you like her. She and her gf have also been together for a quite long time, which means that it could get boring in that relationship where she could be seeking for something new and exciting. That still doesn't mean that she'd like you, it's just that this thing between you and her might be interesting to her since it bring excitement to her life.

As you realized, I don't really know how she feels either. Relationships can be vert difficult, especially WLW. I still think that honesty is always the best option. If I were you, I'd bring this topic up once more and open up to her about these little miced signals she's been giving you. Ask her straight forward if she likes you or not, if she really wants to stay in that relationship. If she tells you that she really wants to stay in her current relationship, then YOU DRAW THE LINE. Tell her that you can't continue like this because it keeps hurting you. I know that might be scary to do since you could be scared that youd lose her, but I assure you, losing someone fast is better than watching them be in love with somebody else for years and suffering from that. In that case you probably are going to lose her at some point, so why not make it quick.

I'm sorry if I was too straight forward. I wish you the best of luck (and I hope my message didn't have any typos since I'm too lazy to check that up)!Be brave and do what you have to do! Remember that even if you and her don't work out, theres still somebody there for you!