When I was a bit younger, I thought mantras were silly. I thought that chanting the same thing over and over was never going to work; that it was a waste of time to try to alter your way of thinking by repeating a word or phrase. I know now that it didn't work for me because I was so hesitant to let it work for me. Once I gave it a chance, it (surprise, surprise!) started to work for me, and now I use mantras almost every day.
All humans have bad days - where they hate the world, or they feel that the world hates them. We're allowed to have these, but it becomes different when it affects someone physically or even begins to take over their life... much like what I struggled with earlier in my life.
Mantras should be something like steps; start out small and work up! By this, I mean starting with an easy mantra like, "It will be okay today."
Then, "I will be okay today."
Or, "Today will be a good day."
Remember that you should start trying to improve your perception of your situation. Don't overreact. Don't take things personally. Perceive that things are being done either innocently or to your benefit and see what happens. Keep in mind that this will not be an instant or overnight epiphany. It is a process, much like anything else. You will find that improving your perception thus improves your mood and your outlook.
Give it a try, and good luck!