She doesn't cum during sex, never has with any guy, even with her on top, grinding, etc. She says it feels good, but she just doesn't cum. Ok, I can handle that.

However, I can't get her off with my fingers or tongue.

When I watch her masturbate herself, it takes her at least 15 minutes to cum, even when she's really turned on (like we've just been having sex). She gives herself constant, direct, clitoral stimulation, always with the same motion, only pausing briefly to lick her fingers as lube.

I try to do the same thing, but I sometimes can't maintain the same rhythym for that long, and sometimes I just inevitably slip off the clit or stop rubbing it in the exact manner she likes. I always go back, but she says her arousal plunges quickly if things change down there.

I've tried eating her out, and while a lot of things down there feel good (kissing, nuzzling, grinding), the only thing that really makes her feel good is flicking her clit with my tongue. However again, I can't maintain the same motion in the same manner for 15-30 minutes or longer, due to endurance of my tongue, shoulders getting cramped, etc.

I've tried giving her additional stimulation in ways she can't easily do herself, such as rubbing her g-spot and deep spot while I rub her clit, and she likes it, but it doesn't speed up the amount of time it takes.

I'm at my wits end. I feel inadequate when she always has to take over to finish herself. Help!!