Premature, forcible retraction of an infant's or young boy's foreskin can indeed produce scarring, sometimes resulting in artificially induced phimosis, as well as other damage. But you don't seem to have that problem, Matt, with that beautiful wide 'skin of yours...nor would phimosis alone cause decreased sensitivity anyway.
I don't know of any method of actively increasing the sensitivity of the glans, but it can be "passively" improved by keeping it hooded at all times and avoiding abrasion against clothing, and too much or too forceful manual stimulation of the bare head. (There was another thread not too long ago by a circumcised OP asking this same question.) If you don't "literally last for hours" solo, but only with partnered sex, most likely you have no residual damage to your glans or inner foreskin that might cause your problem. If it does apply to masturbation, then possibly you should consult a good urologist, who might in turn refer you to a nerve specialist for an examination of the entire nerve system governing the genitals.
But to begin with, a few speculative suggestions: (1) Use your nondominant hand when you masturbate--will require adjusting to a different feel and technique. (2) Use a little lube under your foreskin and on your entire penis when masturbating or being masturbated--will accustom you to a lighter, slipperier touch and give your foreskin more mobility for stimulation. (3) If you're used to retracting behind the glans during masturbation or sex, try keeping the foreskin as far forward toward the tip as you can (you seem to indicate that you automatically retract partially upon erection, which is quite normal), with inner lubrication--will give gentler, steadier stimulation, perhaps avoiding possible nerve exhaustion in the glans. (4) And most important, experiment with frot, alternating at intervals between one of your hands and one of your partner's--will "wean" you gradually away from reaching climax with your hand alone, with some of the stimulation coming from your partner's. The transfer to his alone after some time should follow.
You don't say what you and your partner's sexual activities are--so in addition to applying the above suggestions during sex, give your imaginations free reign to devise any and all possible means of bringing you to the highest level of erotic excitement and tension. This assumes, of course, that you are both completely at ease with each other sexually--if not, there may be a psychological block at work, since you climax with solo masturbation. Think about this aspect, too--just possibly your problem lies there...?
Good wishes and best of luck to you, buddy. It'll work out sooner or later.
