We have the control to shape our minds, to be positive or negative, to what we want to be. Our thought process reflects on our personality. You can't pretend all the time. So what you are thinking about yourself and others actually matter.
It's not like it will become true if you think 'you are stupid' but you surely will start to believe this. And if you continuously tell yourself 'you are strong, you will feel that way. In that sense, yeah, it will become true. Let me share another example. Maybe you are thinking negatively about someone and you usually pretend in front of them to be positive about them. No matter how much good a real-life actor you are it will reflect on your way of behaving.
The thing is, what we put in our mind just process. So it's our responsibility to notice what we actually want to preserve there. If you want to feel confident and beautiful you have to tell yourself that. If you want to be a positive minded person, you have to think like that. It will never work if you tell yourself the opposite and you will act as per you want.
Remember, if you think something negative about somebody it doesn't make them negative, no matter if they really are or not. But it sure makes an impact on yourself and your mind will act like that. We always can't control our response and reaction. That's why realizing this is very necessary. And once you understand how this thing works, you will feel like having the key. Because you surely will start to think more positively, more good about yourself.
So, decide what you want to tell yourself and notice what you are telling.