You can be successful without mentor if you are the type of person who can control himself.
Controlling yourself means that you can trade by trading rules you have set or someone have set them for you.You will not brake those rules in order to stay in the game.
Mentor is helpful a the start where he can teach you stuff you would learn by yourself for longer period of time. He can show you tricks you would learn later on in the future.
He would shorten the learning time.
Mentor in the later stage of trading is helpful if you are not disciplined and then he can help you to follow your trading rules by watching what you do and is that inline with the trading strategy.
He can tell you that you should not open or should open desired trade because it is based on trading rules. All of this is not necessary to have if you are disciplined. But many traders are not disciplined and they jump into trade without trading rules even they have them.