“This too shall pass.”
It seems like a very simple idea, but there is so much built into it. Everything in life is transient, and fragile so should not be taken for granted. When things are going well for you, you should be grateful, you should actively appreciate it because it will not last forever. When things are not going well, you should hang on and weather the storm because this too shall pass.
If you can hold this phrase at the centre of your life, it will not allow you to be complacent, or ungrateful, but equally in will not allow you to give up or fall into meloncholia.