Welcome to H.C.M.
( hand job) is perfectly a normal thing to do. You do not have to feel guilty about it. As for the feeling that your penis is getting small and your penis nerves are getting weak, it is not true. Masturbation is not going to make your penis small or weaken it's nerve strength. You do not have to worry. If you have an urge to masturbate and settings are right, go ahead and do it and do not feel guilty about it. The urge to masturbate is a good sign that your hormones are functioning properly.
The idea that masturbation will make you weak is absolutely wrong.