Well im not cut either and my foreskin while erect only covers half of my penis and i have full sestivity so i think it may be a medical problem because i seriously dout if mastabation, excessive or not, would of cause your penis head to "dry out".
If you are talking about your foreskin only covers half of the penis head while your are
NOT hard then you could have a problem there. It should be longer than that but you shouldnt have lost sesitivity because theres is plenty of sestivity in guys penis's who are fully cut.
If the problem is that your foreskin is "too short" then you could research foreskin restoration or talk to guys who have restored their own from having being cut off while they were a baby, the same techs they use can stretch the foreskin you allready have to give you your desired length. And dont worrie about it stretching too far scientists estamate that one foreskin can be stretched to the size of a football feild so you have nothing to worry about.
Hope that i have helped you