You get a lot of self-help that talks about putting negative thoughts out of your mind and replacing them with positive ones, but I don't think this works. If you hide your negative thoughts, you basically bottle them up, until they accumulate and explode like a dam bursting... and you don't even understand why you are over-reacting to something quite small (the final straw) - you don't recognize everything you bottled away.
So it is important to process your negative thoughts and really feel them, before you then try to be more positive and look for the good, and opportunities, in your life situation...