To an extent, it's a problem that will lessen over time.. How long have you been having sex? The longer you're active, the less you'll be so-horny-you-can't-hold-it the second it starts..
One way to train yourself is when you're whacking off.. You say it only takes you a minute, but you're probably just going full-force until you blow. Don't do that. Go until you can tell an orgasm is coming on, then stop and let yourself cool down. Start again until you feel it coming on, then stop again and cool down. Do that a bunch and try to stretch out the time of your solo sessions as much as possible (after a bunch of near-orgasms the mental urge to finish will probably be too much.. it will also be much more satisfying which is always fun). As you get more practice with that, change the routine up so that instead of stopping completely, you just change your rhythm when you feel an orgasm coming on. When you're really comfortable with that, you'll be able to do the same thing during sex - feel when an orgasm is coming, slow down, keep your cool, and go longer.
And yea shooting one out an hour or two before sex doesn't hurt either.