When you masturbate try to make it last longer by stopping right before you bust. You have to learn your thresh hold.
Next time you think your going to have sex mastubate several times that day and try drinking a beer or glass of wine right before hand.
Are you nervous when you have sex with her or relaxed? The more relaxed I am the longer I last.
Also more sex would help. See if she will start doing it everyday a couple times a day. She probably won't but it's worth a shot (tell her you need to be trained). Once you get good at sport screwing, making love is a breeze. I've went full circle from coming way to early (sometimes during foreplay), to not being able to get off sometimes at all and having to fake it just because the women can't take anymore pounding.