Maybe you’re not physically attracted to your girl anymore. I’m not saying you’re gay, just that maybe she doesn’t do it for you anymore.
Maybe you just have a low sex drive in general though. I’ve heard of some guys just not having a lot of sexual vigor. Even young guys can experience this.
I would at least get an opinion on it from a trained medical professional though, regardless of the advice you get here. You need to do as much as you can to figure out why you can’t O.
Btw, some final suggestions: Don’t wack off, don’t watch porn, and try other things with your girl other than vanilla sex. Ever tried anal? That’s sure to make you cum. Boredom is what kills sex drive, so keep it interesting and try new things. =)