Can't Ejaculate And Not Into Sex

i'm 20 years old, 21 this year. I've been sexually active since 16 and haven't gone more than two weeks without sex as I've had about 4 different partners.

Since 16 I noticed, or read, that most 18-21 year olds can have sex 2-3 times in one day. I can only manage once in a 24 hour period. It also suggests that men of this age should be able to have sex at least once a day but I seem to be able to have sex twice or three times in a seven day period and have to wait another three days before I can have sex again. Its frustrating because I can get an erection about two hours after and really want to have sex but as soon as it gets going it's almost like I just can't be bothered, even six to eight hours later. It also frustrates any partner I've been with a they're previous partners were all okay to go two - three times in one night. I've been to the doctors twice now and they send me away saying it's something stupid like thrush (which they've tried to treat me for already - it can't have lasted 5 years plus I have no pain or discomfort).

Recently as well in the past two weeks I've suddenly gone off of sex completely. Or when I do feel a slight urge to have sex with my girlfriend we get seven minutes into it and I can't ejaculate or even feel the want to. I've gone through boxes of condomsm with no success. Its very frustrating for her now and of course myself. I just don't know what to do this time. I have no extra stress in my life, I've had a slight illness but nothing which would affect sexual performance as ive had this virus for around four weeks and I only lost interest in sex two weeks ago. I just want to be a normal 21 year old!