Miles, I am really sorry you are going through this and that you are in pain.

I am of the opinion that if you really love someone, you want what is best for them and for them to be happy. Despite your feelings about the situation.
Despite what you think you think or know is best for them.

If your girlfriend is miserable, IMHO, let her go. Tell her to leave. Encourage her to be happy and to do what is best for her. But also, make sure that you also put a timeline on things and make it very clear that you will not be waiting around forever. You'll need a type of closure so you can heal yourself. It's not easy (probably one of the hardest things you will ever have to do). But you can love her with everything you are made of. But you also (have to) love yourself just a teeny bit more. AND you also mentioned that there are a million reasons why she should choose you over the other person ... that is NO way to be in a relationship...it's crazy-making because love, attachment, sickness, mi, dynamics, etc complicated and don't make a bit of sense.

It's a cliche but it's true ... set it free.

Both you and your girlfriend will be better for your strength, fortitude and the strength that you show yourself.

She sounds as if she is in no position to make any more serious decisions. And you enabling her by being there is not helping you, her or the boys. at all.

I truly don't mean to sound harsh or critical. I am sincere in that I think you should take care of yourself and let her go.

Best wishes to you