Happiness depends on a measure of inner contentment, and also fulfillment. And there are some distinctions here we all need to understand.
Contentment is simply accepting the present as beiing what it is—letting go of bitterness, resentment, anger, frustration, and the sting of all such negative emotions and attitudes. It doesn't require liking all that's present, or wanting it to continue, or even not planning to work hard to change things radically from being what they are to what they should be. It's merely a shedding of those negative emotions that get us all balled up and unable to move forward productively and well.
The person with a measure of inner contentment finds things to focus on emotionally that are good, and that can be a basis for renewal and enjoyment and moving forward with whatever hard work needs to be done, A moment, or stretch, of intense discontent can motivate mightily, but extended, it begins to get in the way. You can set your heart with passion to work for a better future without requiring the ongoing fuel of anger or hatred. Those things are inwardly corrosive over time and tend to guide us to replace one unfortunate state of affairs with another, however well intentioned we might be. For more on this, see the history of revolutions.
Contentment says of the unpleasant, unfortunate, and unjust in the moment, "It is what it is." It takes a deep breath, gets back in touch with the best of inner motivations, like love and compassion, and goes to work with that oasis within needed for the longest stretches of travel through any arid desert.
Fulfillment is something different. It's experiencing an ongoing objective realization of your potential, and feeling that inside. It's objective before it's subjective. Whereas contentment is just an inner state, fulfillment is something in the outer world that reflects itself within. Put your talents to work for the good of others as well as yourse. Be a blessing to others. Work hard for justice and goodness and truth. And fulfillment will come, as long as that enemy of lingering discontent doesn't get and stay in the way.
Contentment and fultillment: Two foundations of that deep Aristotelian happiness that promotes excellence and full human flourishing in all things. May you experience both, in even the most trying of times.