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Try getting hard, then putting on a condom and jacking off in it. Get used the idea of stopping, putting on a condom, and carrying on again.
Yep, I agree with this completely. I had to go through a fucking personal training session using this method because despite not being a virgin, I still had the softy problem a lot.
Also, this is kind of awkward, but practice masturbating in sexual positions. I would always go soft if a girl wanted to go on all fours, so I masturbated in that position since there's a huge difference in your body's orientation, particularly where your legs go. Practice all kinds of positions with the condom and you'll get used to it in no time. Also, try using lube. It'll cause less friction than your hand which is similar to the vagina, which increases your sensitivity and makes it easier to get aroused.
To go along with what Fuze said, I agree that any kind of math is helpful for relaxing you. I like to do calculus since that always kicked my ass when I was in school, so it takes some serious brainpower to do it, especially without paper. By the time you solve the problem you'll have been fucking her for 5 minutes without even realizing.
If worst comes to worst, I've found that having 2 beers - no more and no less - is enough to get you feeling pretty good. And if that still doesn't work, have her give you a blowjob for a few minutes, then go for it.