It all has to do with TOO much masterbating. You trained your **** to cumm to your hand for TOO many years. Its what he knows best. You need to stop masterbating for a while. Completely stop, if you don't now, your going to have a long line of unsatisfied women.
Just because you can pound on a women for hours doesn't make you a good lover. Women will develop complexes if you don't come. For women, sex is almost as much about pleasing the man as it is getting off for themselves (especially in relationships)
Plus whats the point of picking up a women at a bar or for a ONS if you can't orgasm. You might as well stick to your palm for the rest of you life. Is that what you really want?
If I were you, I would stop masterbating for at least a month. I'd also cut out sex with your g/f for about 2 weeks, however in the meantime, I'd still make out and get yourself hard, just no penetration, no hand jobs, no blow jobs. You want to build up the sexual tension so that in 2 weeks when you go ahead and blow your load, it will blast off.
If you can't go without sex for a couple of weeks to fix your disfunction, then you've got a lifetime of discomfort coming up. Personally I'd much rather make a short term sacrifice.
Also cut down on masturbating (if not eliminating it entirely), and you'll be able to get off with your gf more easily.