The masturbation timing sounds fine but if you want to last longer during sex, try this:
Loys of foreplay till your gf is almost ready to orgasm. let her get off first, when you feel the urge to orgasm yourself, stop for a bit to take away the heat but keep stimulating her. Sart again & repeat till you want to do it. And try to keep going afterwards, practice being able to come more than once.
Masturbate an hour before sex if you know its going to happen. That way its not such a huge deisre to orgasm straight away when you do have sex.