I don't think I agree that if you don't believe your hair will grow, it won't. If you take care of your hair then, barring some health or scalp issue, it's going to grow regardless of what you think. I'm pretty sure about that.
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
But I do agree, as someone has already said, that if you believe your hair will grow, your more likely to do the things you should for it. Plus, being positive makes the journey so much better.
I'm writing this because I know there have been times when I didn't feel positive about my hair or that I was doubtful and that was difficult enough. But then I got worried that these feelings and thoughts would negatively affect my hair and that made everything ten times worse. (Btw, I'm pretty sure my hair kept growing throughout all of that.) It's normal to have doubts and negative thoughts. I think its important to make a commitment to just continue to take care of your hair and try to be patient, even when you feel like you can't reach your goals.
Now that I've said that, I've always done a lot of visualization. I imagine myself with my hair at my goal length. I think that's really helped make me more patient and motivated about reaching my goals.