my penis goes soft during sex im 23yr old so wanna know if this is normal and what i can do solve it

hey im a 23 yr old, im fit and healthy i work out everyday i have a 6.5 inch penis and just the other nite i was having sex with a girl i just meet and we were having sex everything was good but then i got to the 3rd round and my penis just went soft so she was sucking it for awhile till get got hard and just before i could put it in it just went soft again so i was just pleasuring her for awhile then my penis was so erected so i put it in her and it was very quick then he went soft again, it was 3am at night so could this be cause he tired and had enough? cause this not the only time this has happen, but i think my mind goes all over the place sometimes so that could be the reason, i wanna have sex so bad but he doesn't lol