What Causes Loss Of Sensitivity In Penis?

i am 34yr old male for the last 20 yrs i have been masturbating on average once a day, i've never had a problem getting an erection or having sex that is up until about 18 months ago around the same time i lost a lot of sensitivity in my penis and noticed that it was soft and limp more so than usual when not erect, have i damaged my penis from excessive masturbation? i am in a steady relationship now