Penis will not fully erect and ejaculate as much

Patient: Hello, recently while trying to masturbate and after trying to have sex with my girlfriend, I found my penis would not get as fully erect as it used to. When I had or almost did have sex with previous girlfriends my penis would become fully erect with no complications. I am very sexually attracted to my girlfriend but felt nervous when we first had sex. I also grew a tendency to masturbate twice and even three times a day before we began having sex. In addition to not becoming fully erect, i noticed I ejaculated less and after a day or so I still produce less volume than what i used to normally even when masturbating twice. Am i ejaculating less because I am not getting fully erect? And is my penis not becoming fully erect because I am masturbating so much?

Doctor: