>>33308894cont'd
Having more sex will give you practice and increase your stamina. Look up edging as well, the practice of bringing yourself right to the point of orgasm and stopping all stimulation to prevent it, then continuing. You can do this while jerking off and apply it during sex.
You cana have your gf give you a blowjob before penetrating her, either have her finish you and do the hands free thing i mentioned above or just have her do it and edge as much as you can before putting it in. It will make good foreplay and you won't make her feel like she's doing some "task" to help you, it'll just be part of sex.
Condoms can help dull the sensitivity of sex, but I argue that if you can avoid them for the purposes of getting good at sex, you should. You miss a lot of the subtle but distinct movements of the vagina as you're having sex that give you the feedback necessary to give a woman an orgasm. Until you're good at it, making a woman cum with a condom on is hard. But, it is a valid way to last longer. Personally if you're in a relationship and it isn't otherwise necessary, just go raw and practice as I've indicated.
Penis stimulation is about friction and contact. If you stop the friction too late when the orgasm has begun, the mere contact you have with the vagina will cause you to nut. This is why kegels are important, they will help you pinch off the orgasm when the stimulation has gone beyond your control (very helpful in cowgirl and other positions where the woman is in control and continues to move while you're trying to stop movement to prevent nutting). You can actually make this a game with your woman, get hard, stick it in, and try not to cum while she tries to make you cum. Keep scores, make sexy bets, whatever you want. Just stay hard and try not to cum without pulling out. If she can make herself cum first or gets tired before you nut, you win.
Try to focus on the feeling you get immediately before you nut and recognizing it.