You mention that you are currently seeing a therapist. I think that it might be helpful for you to bring this issue up with your own T. They should understand abandonment issues and be able to help you find a way to break up with your girlfriend in as gentle a way as possible. This might take a few sessions, though...

Also, have you thought about writing her a letter? I would try writing everything out, so that you make sure you get all your points across -- such as it not being healthy for either of you, your concern for her safety, wanting to do what is best for the both of you, etc. Perhaps you can take this to her and she could read it while you're still there. I want to suggest having an open discussion with her, but I feel like she's not much for discussing when she's feeling rejected or abandoned.

I would definitely ask your T for advice.

Good luck!
,
Ro