When having sex with my girlfriend I don’t seem to

Patient: When having sex with my girlfriend I don’t seem to ejaculate, we have had sex 6 times now and I am still yet to ejaculate. However when I masturbate I can make myself ejaculate quickly, im just wondering why this is as it is very annoying that in cannot ejaculate through sex with my girlfriend, as usually I have to finish myself off as I don’t cum from sex, I believe it may be because of my masturbation technique that my body has adjusted too and when I receive something different do not ejaculate, im not under any stress or taking any medication/drugs so I don’t understand why this is occurring, do you think stopping masturbating would help? Thanks.

Doctor: