er, um it is impossible for any male to jackoff too much ... once your body is satiated, you will discover one of two things ... an inability to get hard, or an inability to ejaculate any fluids.
your biggest sex organ is your brain.
I confess some surprise about your condition, as I was under the impression that natural curiosity would intervene at some point in your life, and you would have tried to see how many times you could make yourself cum when you were about 14 years of age, when puberty provides you with about three times the amount of testosterone of a normal adult male.
There are numerous possible reasons, such as testosterone level, previous trauma (injury) to the penis, or nerve damage caused by Peyronie's disease, or diabetes etc. in addition to the suggestions already posted ... zoloft, in particular, was an antidepressant with the unfortunate sideffect (depending on your view of course) of making it possible for males to pound their partners for hours on end before ejaculation or orgasm could occur. Others claim similar sideffects due to Viagra, Cialis, etc. There are various other meds (high blood pressure, etc) that can alter your body's responsiveness ... and we have not yet opened up the Pandora's box of over the counter medications, and their side effects and interactions with other substances.
Generally speaking, it may be that you are simply unique. I once had a girlfriend in highschool who announced to me that she could make me cum in less than fifteen seconds ... when I asked her to prove it to me, she took up the challenge ... in a totally dark room (no distractions) with me naked on a long towel on the floor. She did not let on that she was using olive oil to lube my entire genital area, and proceeded to lovingly stroke my hardon, all the while telling me how huge my cock was ... she then concentrated her hands on my frenulum (just underneath the head, on the lower side of my cock ... and then made her hand into a cup shape, added oil to the head of my cock, and swirled it mercilessly with significant pressure while holding my shaft with her other hand ... I blew a 'petite mort' orgasm that wrenched and convulsed my body for what seemed an eternity seconds later, spilling more sperm than I had ever done previously.
Give it a try.