Sex Issues

Sexually inexperienced. Im 19, this is the first girl i've had sex with. She is really hot, we've been dating for like 4 months, and the sex is great sometimes, but other times i just lose my boner. It feels great, but it takes me like 20X longer to orgasm w/ sex than by say masturbation. Sometimes it takes too long, and I lose my erection. Sometimes I dont, and we go for an hour or so, break, and do it again later that day.


Any suggestions? Serious please.