Stubbornness is also determination. It's simply a matter of shifting from "won't power" to "will power."
The simple solution for disappointment depression: Get up and get moving. Physically move. Do. Act. Get going.
Acceptance is such an important commodity, some have called it "the first law of personal growth."
Acceptance is not a state of passivity or inaction. I am not saying you can't change the world, right wrongs, or replace evil with good. Acceptance is, in fact, the first step to successful action. If you don't fully accept a situation precisely the way it is, you will have difficulty changing it. Moreover, if you don't fully accept the situation, you will never really know if the situation should be changed.
If you want peace, stop fighting. If you want peace of mind, stop fighting with your thoughts.