Thank you so much for your mail.
First: the girl. She’s shown you (almost) beyond doubt that she’s in a relationship. The fact that her partner is not around makes her miss certain things that she would do with him if he were to be around. And that’s why you got that kiss. Her feeling guilty and telling you is a sign that she values her relationship with her partner more than what she has with you.
Now you: how would you feel if your girlfriend was kissing another guy because he’s close enough? Yes, put yourself in his shoes.
And then, think of how the relationship will be in future if you can’t put yourself in his shoes. Would you be able to trust her?
You can make the decision to stop these feelings for and keep your distance for a while. Give her the space to make a sincere decision on who she wants to be with to avoid your being used as the stand-in boyfriend.
All the best.