Losing erection quality fast nonsensitive dick after some sex?

Hi all,

I don’t have that much sexual experience, but I notice that with my girlfriend and also when I masturbate I lose my erection quite fast. Lets say I get fully hard, and then I would not do anything with my penis and just watch tv for 15 seconds then it already loses its hardness really fast.

I also have a general question about sex. Like when I have sex with my girlfriend the first 5-10 minutes or so I can fully enjoy it, but then it just seems like my dick is getting too exercised and I start losing feeling. It happens to the point that I will start to go limp (after around 15mins depending on how much I had sex before), and that I am no longer able to come.

It’s like I’m very horny at the beginning, but the physical exercise and the sex makes my penis not sensitive anymore. I barely feel I’m fucking someone after some time..

With my girlfriend we just do it with condoms, and I’m uncircumcised and the condoms also seem to play a part I think, but I’m not sure. I wanna try without condom but girlfriend is not on the pill and obviously don’t wanna risk anything.

Thanks for helping!