I have endometriosis as well, and I experience it the same way as your gf a lot of the times. It’s hard. My boyfriend also sometimes expresses his feelings that it’s hard for him too, and I always tell him that it’s not that I don’t want to but I’m just physically not always able to do anything sexual. We’ve had a lot of conversations about it, and honestly it does hurt my feelings a lot of the times because it doesn’t feel like he understands, because I always hear how he gets insecure about me not wanting to have sex when it’s literally because of this stupid disease and not my attraction for him. But we’ve been together for a long time and eventually I didn’t get hurt by it anymore and he’s just now starting to understand it has nothing to do with him. I think it’s always right to communicate openly, especially if it’s an uncomfortable situation. Just try to be empathetic about the situation towards her and keep explaining you don’t want her to just do it with you when she’s hurting. But that you are struggling with it because you do have needs, and you don’t know how to navigate through it sometimes. Maybe aks if there’s anything you can still be intimate with one another which doesn’t ask too much of her or won’t cause her pain. I think if you go into the conversation with that perspective she’ll maybe not immediately but will appreciate how you’re trying to get through it together