You can make this question as simple or complex as you want. Happiness, in its traditional sense, is influenced by so many factors.
But back to basics, what is ‘happiness’?
I believe the ‘happiness’ we seek needs to be clarified and distilled into a more practical form. The better term is ‘neutrality’. When striving to achieve happiness, we should aim to be ok with the current state of things. We shouldn’t aim to have an emotional state above a neutral baseline. Being elated or ecstatic are temporary, as are feelings of depression and anger. ‘Happiness’ is the state of accepting the way things are.
Based on this definition, I am not happy. But I am also not depressed. I just wish things were different. I wish I had more money, I wish I were taller, I wish I had more mental focus etcetera. I don’t yet have faith in my own mental fortitude to entrust my ‘happiness’ to simply shifting my mindset. That is to say, I don’t believe my ability to accept things is developed enough to provide me happiness. Maybe it will over time, but not yet. Thus my emotions are not neutral and I am, per the definition above, not happy since I am longing for a different world than the one I currently live in.
As I’ve alluded to above, I believe happiness ultimately comes from within. Happiness comes when you are ok with not achieving your ambitions or being someone different to who you currently are. The key to happiness is acceptance. This is my belief as it is what I’ve read and briefly experienced (I think). But I can’t speak on this topic with absolute authority given I haven’t achieved 100% happiness.