seems more psychological than physical, forget about the problem and enjoy the sex. The anxiety is putting pressure on you to cum which is doing the opposite, ask your gf to surprise you at random times for sex e.g outdoors etc if shes adventures, or maybe blindfold one another, take turns and let them do anything. Its all to do with hhe mind than physical, in your case, im no doctor though.
Happened to me, though i was slightly bored of the repetitive sex and was always the last to cum, still am but i can still cum during sex, just takes awhile.
Like i said, its all psychological, if its not fully turning you on then you'll struggle, sex for the basis of sex could be a problem, try making love and DON'T think about you, think about how amazing the experiences is. Try getting her to give you a bj with a glass of cold water, drinking some to make her mouth icey, then sucking the head of your penid. If she aint into oral, you could get some cock rub and just relax. You could also try asking her to maybe do some things you've fantasized without making her feel normal sex aint good enough, all depends on your partner's willingness and you letting go, question is, do you feel anything? Are you enjoying it? If your missing one of these, you will struggle to cum inside of her. (you tried pleasure enhancement condoms? They can help alot if you're struggling bare back)