For many people, contentment is the true measure of success. At the very least, recognition of those areas in life where we are content allows us to concentrate on other areas where we are not.
A good starting point is with gratitude. Give thanks for all that you have, not only in material terms, but also for your security, your health, your relationships and so on. You may like to make a list and add to it from time to time. Its length may surprise you!
If the contentment you seek is in connection with a situation you feel you cannot change, yet that situation is hard to live with, the question to ask yourself is, "What opportunity does this situation present?"
In the big scheme of things, there are no losers, every situation presenting opportunities for all involved to win. The difficulty is usually in spotting these opportunities whilst feeling in adversity.
Ultimately you have to make a choice between letting go or making changes.