Each time you masturbate, you are reinforcing that idea in your head that this is what you need in order to climax. Your body doesn't know when your girl has a headache or isn't around or whatever. It doesn't know why you're beating off and doesn't care. It just knows what feels good and jackin' feels good. The more you do it, the more used to it your body (and mind) gets and it gets interpreted as the norm for how you like to have sex. Since a human mouth or pussy feels a lot different than a dry hand (assuming you jerk dry), your mind is gonna say "Hold up! What's going on here?". And even if it's her hand, it's still gonna feel different.....because it is. That's my theory, anyway.
If I had a girl, she's probably hate me after a while because I've spent so many years jacking off that I doubt I could really get off any other way.
But ask yourself this. Is it your fault? No. Men jack off in the absence of some other form of sexual outlet and sometimes, the latter just isn't available or possible. Real sex can be scarce for a man so it isn't surprising that this is a common issue. So, if it is a psychological thing, don't beat yourself up over it. Just try and find ways to deal with it.
I would start by trying to cut way back on masturbation and save all that energy for her. It might not kick in immediately but work at it and see what happens. Some people need time to adjust to a new partner or different feelings or sensations.
If you must continue masturbating, the best way would probably be to use a good quality lube and go a little light on the touch. I've had good success with using disposable latex gloves...that way I don't get so used to my hand feeling my dick as I climax (The latex barrier dulls the sensation between my hand and my dick and is more like fucking something else).
Of course, as always, if nothing else seems to work, you'll want to see a doc and see what he says. As long as you can still cum somehow, I wouldn't worry too much about it being a physical problem.
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