Taking a long time to ejaculate with partner

I'm a 18 year old guy here who has no problems cumming while watching porn, but when me and my lady friend hang out, it takes me over an hour to cum. We don't have complete sex, she jacks me off and gives me head. It always feels really good and sometimes I feel like I might be orgasming but not cumming for some reason. Might I add that this is done in the backseat of my car and I can feel my legs fall asleep while we are going at it. I know I'm turned on while shes doing it, but just take a long time to climax. When I have actual sex in a bed though, I am able to cum just fine. Has this ever happened to anyone else? Does anyone have any reason to why this might be happening? Thanks