Good day, Dear Dr Richards.
Just a few days ago i noticed that erection during mastrurbation was weaker than usual. Not too weak, but now penis is erecting much slower. I haven’t seen morning erection for 4 days.
I’m almost 21 and have been masturbating since 18. Since that time i had several sexual intercourses with women and didn’t experience such problems.
I want to ask does these symptoms mean sexual exhaustion and is it possible to restore my erection without using medicines just by stopping masturbating for a month or more and changing lifestyle?
I live in Russia, so it is too complicated to purchase medicines that you usually advice in similar cases.
I’m looking forward to your answer.

Since there are no other symptoms, I would conclude that you have temporary burned much neurochemicals needed to support sexual functioning. This is quite dangerous and may turn into a chronic case of sexual exhaustion if repeated.
The best way would be to power your activities properly.
Take a break and make some positive lifestyle changes, such as: proper diet, exercise, massages, and positive attitude.