I can make my girlfriend climax, but not anymore

That's how she describes it; She gets that amazing feeling when you're just about to come, but it just keeps going, with no actual release.

I am her first sexual partner, and I have managed to make her orgasm properly once, but never again since then.

Now, she says that it's okay and that I shouldn't worry about it, but I really want to make her experience proper ecstasy, and it worries me that she can't.

I can't imagine how frustrating it must be for her; Having that feeling of "I'm gonna come" but never actually getting there. She once had to stop me because it was just too much.

Any thoughts would be appreciated!

Thanks.