I've always liked bittersweet stories and endings, but it doesn't seem like something easy to do. Many films for example seem to try to do bittersweet by having the main character succeed but unfortunately die, or not succeed but grow in character or such. I've found those to be too easy and obvious and not very bittersweet anymore. The things I have found to be bittersweet(which isn't often) don't really seem to have much of a formula but just are for some reason, so is the bittersweet feeling just maybe different for everybody?
Maybe the mc having what he wants and dream of within reach but giving it up in an act of unselfishness for the benefit of someone else or for a bigger purpose? I find this hard to do too, but more because ending a story like this would make ME as an author feel unsatisfied. I like the happy ending. Not necessarily "mc gets everything she wants and needs and every problem is solved to her advantage", but rather hopeful endings where readers can fill in the rest for themselves.
How emotional something is or how obvious/cheesy it seems obviously depends on the person (e.g. your average rom-com might get one person crying at the "beauty" of it, but someone else laughing about how forced and cliched it is). Maybe try thinking about the books/films you like and try to think about what makes them work and what you like about them.
I think every quality happy end is bittersweet. I can think of so many famous stories like that. I think it's best to analyse those you do like, and draw conclusions about how it's done each time. Like you said, everybody's perception is different.
I read somewhere once that a powerful ending revolves around sacrifice. I think that is a good thing to remember in this case.
The bittersweet really hinges on your feelings towards the character(s) involved. A well told story with likable characters can have a successful bittersweet moment even if that moment is obvious or unoriginal. Yet if originality is what you are searching for then there are options.... The bitter and the sweet don't have to be the same moment.... The moment doesn't have to take place at the climax of the story.... Always.... and I mean always know that to have the most profound effect... skew the ratio to favor bitter
I just wrote a scene where a young boy gets a spanking. A common occurence in the time of my setting, and it did not indicate an abusive parent. Given, kiddo wasn't heard, and he was in trouble for hitting a girl even though the girl deserved it, no one wanted to hear him out because of the elusive "hitting a girl" got more attention. When I finished the scene I was struck with how precious it was, and how worn both father and son were from the event. How the eventual listening afterward caused each to break, and upon re-reading it, I was moved. (Yay me!). It's not that any "good" came from it. But life lessons learned, I guess? The next chapter I did end up making a slight reference to it when the mother told another child she would not be in trouble for explaining what happened. So maybe there was character growth, but it was unintentional.
