Yes. Every day I see proof of it in my own life and in others.
For example, I was scared out my mind to write for money. I thought that I wasn't good enough, so instead I blogged in my own little world and kept on a track that earned me very little. Eventually, I decided that I needed to start making some money and I changed my focus towards ventures that would make more money, quickly (which meant interacting with others and putting myself out there for rejection). I also started believing in myself and my ability, and guess what - I succeeded.
It's been like that in every area of my life. If something is not working then you have to push your comfort zone and change your focus towards something else. And your mindset or attitude have a huge influence over whether you will succeed with your new focus.