Our state of mind affects our happiness a lot, because if we think about it, the way that we feel about different things comes from our own perception of those things, for example if we think that making a mistake is really bad, if we actually think about the mistake we have made under a different light then we can actually start to see how making that mistake wasn't the worst thing in the world and that you actually learned something from that mistake and that it all worked out in the end. Happiness can be created by us, we can make ourselves happy, it's all a matter of perception.