In fact, I recommend people to do that. For example, before your semester presentation, do to a corner and tell yourself, you can do it and you are going to impress the teachers in your presentation. Successful people do that constantly i read somewhere. They encourage themselves, give themselves surprises and presents as forms of positive self support. When they face problems, they do not falter easily. Their positive self-talk give them much confidence and prod them to move forward bravely.

This is one way to push yourself on, so that in really adverse time, when you are alone, you still maintain that fire in your belly to drive on, so sticking to something you passionately believe is possible.
Believe in yourself positively. Have a firm grip of your mind so that it works for your positively. Yes, positive self-talking will change your perception of yourself.