I think what makes me happy/content in a long term way is trust...trust in me, my abilities, in life (call it destiny or whatever supernatural energy there might be)...walking new paths and, by ignoring (ir)rational fears, learning that I can manage it, that I grow and that every new experience makes me more trusting step by step... kind of like 'everything will be good in the end. And if it isn't good, it's not the end'.
Also I try to not regret things I've done (even things I'm not proud of having done) but to accept...to accept and learn. Things that happen and seem huge, unchangeable, like negative endless stories in one moment, can be small and not half as bad in the next moment. Sometimes such things can even be the engine of something good to come...new and a bit frightening but good...
Having spoken of trust, I think another piece to my 'contentedness puzzle' is trust in the people around me! Not being suspicious of unknown people and therefore being distant but being curious and open-minded. That is something that allowed me to have experienced several special moments with special people and those experiences make me believe that the majority of humans are good at heart which again makes me feel content.
And also I appreciate doing things I love. I know there are duties which are not as much a pleasure as others and that life does not exclusively consist of fun and awesome activities...but that's why I think it is important to deliberately take some time for oneself. For me this is hiking, being in nature, dancing, go to concerts, talk to friends, meet my family...
As difficult as this question is, I think, for me personally, the way to long term contentedness is the *sum* of everything I have mentioned above. And probably including some more things which I am not aware of right now.