Both quality, of life and also personal growth, depends so much on having the right mindset. If you haven't already I suggest you look up "Growth Mindset" by Carol Dweck.

I think if you are constantly believing that you can improve the state that you're in, and that you can be happier if you try, it has a transformative effect on not just your happiness in the present, but also on how you build yourself to be happier in the future.

Oh, and I'm not sure if the statement "what we think determines what we believe" is always applicable to everyone. You often find people who in their rational mindset understands one thing, but emotionally they want something else to be true so bad that they end up beliving in it anyways.