Replying to Climaxing.... (for lack of a better word) *Note: not for the younger NS crowd*

Yeah, so.. I have been dating this one girl for a few months now, and we've had sex a few times.
But, every time we have had sex, she's climaxed, but she can never "finish me off", so to speak.
Last night, she was determined to get me to climax, and she fucked my brains out for 2 hours in different positions. It was great and all, but I never came close to finishing, and she was exhausted, so she eventually gave up.
She's also given me multiple blowjobs, but she cant get me to "pop" that way either.

Dont get me wrong.. the sex is amazing, and the head is too. She's only had sex once before dating me, so she's nice and tight.
But, for some reason I haven't been close to finishing every time that we've had sex, or when she gives me head.
Now she feels bad about it like it's her fault, and she doesn't know what she can do to get me to climax.. I still enjoy every second of it, but I'd also like to get off at some point. Haha.
In her words, she feels "sexually worthless", which is not how I feel about her, because the sex and head is amazing.. I just cant finish, but I can get her to hit the "big O".

This is not a claim at all, and I honestly cant believe that I am making a thread about such a personal issue... I just was hoping that some of you could share some tips or advice to give to my girlfriend.
The thing is, I've had sex before, and it wasn't even as good as it is with my current girlfriend, but I could still finish. I have yet to do that with the girl I am currently dating.

Any advice? I dont want her to feel "sexually worthless" anymore. Haha.

Thanks.