Sex help

Patient: “hello doctor, i have a serious problem that drive me 20 years old and i used to masturbate since i was i got a girlfriend that shes hot and we like each other like hell.she love me so much and i love her too…but the problem is that i cant cum while having sex with her and this get us upset.i really need help in i masturbate i can cum easily but not when having sex.THANK YOU.