Ok. This is usually not the way it works “expressing your feelings”
You can express your feelings (and you should) if you are in an intimate and strong relationship with her.
Let’s say it’s the other way round. Some girl always wants to do something with you and always pesters you about going out or something. And then she feels you are ready and expresses her feelings to you
How attracted would you be to said women?
Exactly that is the reason why you at the moment don’t have contact with said girl.
I can’t give you “the Tipp” or something “you can use” if there was such thing as the 7 magical words I’d be rich living on an island.
And you did btw not convince her to leave anything open. She said that to get rid of you. Her behaviour betrays that intent.
Maybe she would have been open to cheating on her bf, who knows. But not if you display such low value and go chasing after her.
Go for her friend? She seemed interested enough.