Is it normal for me not to cum with my gf that often

k i have a wonderful girlfriend as of 9months that i love a lot and she does me we always have a good time but when it comes to sexy time im not even lying we have sex for hours and hours we need water breaks and its like that every time but how we tell if were dont is either she cums or we just call it quit gotten lube different positions no toys idk what good that would do me we have fun durning it get kinky we could do a one scence porno but i still just dont get off i dont get it why can this be do i need a doc i dont rarely bother beatin off no more no point i just dont know what to do and i dont want it to turn in the her saying its me isnt it idk guy im lost here i dope you all evjoy your load