So, I'm a bit confused. All the time.
高兴 eg 我很高兴
It seems 高兴 has a connotation of impermanence .. you could be happy right now but unhappy later on. It appears to be the most common, most general form of "happiness"
快乐 eg 新年快乐
It seems 快乐 is used in several set phrases, like wishing somebody a happy [festival, birthday, etc.] but isn't used elsewhere
愉快 eg 周末愉快
I have only heard this used for the above phrase, is it a set phrase for the weekend, and used nowhere else?
幸福 eg 祝你幸福
幸福 appears to be about long-lasting, forever kind of happiness, in life, in marriage, that kind of thing.
My understanding of the difference between these is somewhat fuzzy, and I was hoping somebody would be able to clear it up. Thanks