I can't ejaculate when having sex with my Girlfriend why?

Ok so my penis is so hard and i am interested in having sex but i am used to jerking off to someone i know for years but now i have a girlfriend i don't want to jerk off to that person i want to have sex with my new girlfriend.

i am 25 years old lost my virginity 2 years ago yeah i was a virgin and a nerd but now i am hitting the gym and i am going out and socializing and this is the second girlfriend in two years.

When my girlfriend is giving me oral sex i am ejaculating but a little bit and i can't even feel it i just enjoy my dick being sucked.

when i do it with my hand i ejaculate quickly but when i put it in her pussy or her mouth i don't get the same feeling as when it is in my hand or thinking about the person that i am used to jerking off to.

i tried imagining her the person i jerk of too but i felt like it's cheating. my sex is lasting 45 minutes guys i don't know what to do. i could go more but i don't want to make her tired.

what do i do?