I had a lover who couldn't cum during sex. I was very very understanding about it, and eventually, it came out that he felt like it was expected, and in some way something was wrong with him that he wasn't able to. He was finally able to cum with me because there was nothing at all wrong with him not cumming in my mind.

Allow yourself to be fine with it, let go of the pressure to perform. It's okay either way. Don't think of it as you denying her something, think of it as "just the way it is" instead of thinking of it as a performance. And there's also a thread in the advice area about a person who could only cum while he masturbated because basically he trained himself to do that. Just be patient with yourself, reassure her and yourself that it's nothing to do with her, and it's just an odd thing about you.

Heck, if she ever mentions it at all, just tell her she's SO sexy you can't bring yourself to put an end to your sessions by cumming, but you're trying to get over that.