Maybe you are low on serotonin. Men on SSRI drugs like prozac sometimes have a side effect of being unable to ejac. Serotonin is released when you cllmax, if you are really low it can cause premature ejac problems.
Or the muscles involved in ejaculation could be weak or sensitive. This can be improved by learning to control this muscles. During self-gratification some people practice a technique called edging, where you stop right before you cllmax and take a break then continue until you get that sensation again, then you stop again for a moment then continue etc. This should improve premature ejac. because the cllmax is a muscle spasm that can be controlled with practice.