Sorry to ask a crude question, but why has my penis lost sensitivity

Basically ive used progressively stronger and stronger stimulation on my penis glands, froma  tight grip to rough toys and overly powerful vibrators literally the most powerful ones available on the market. I used these for a long time

Right now my penis doesnt respond to normal stimulation like a handjob, I just cant feel it very much so I dont even gain an erection.

My question is how have I damaged myself on a biological cellular level, would you guess//

Have I damaged the nerves, has the skin become thicker on my glands, has my brain been rewired in someway to only feel a stronger stimulation or has it built a resistance to too much of whatever chemical is released during sexual activity.

What is likely to of happened on a biological level if it is the extra rough stimulation that has caused this.

I have laid off the rough stimulation and sensitivity is slowly returning. Although its hard not to go back to the vibrator much like its hard to avoid eating junk food.  

So whats happening on a cell level, how have I damaged myself