The first one I picked up from a US Navy LCDR about 17 years ago. He told me to never write an email while angry. He said wait for at least 24 hours before replying, noting anything you say in an email will be around "forever". This is probably the single most important advice I've ever received. I'm sure it's saved me from countless trouble over the years.
And this leads in to "pick your battles." I can't recall who I heard it from first. Probably either my grandma or mom. Nearly everyone has heard it somewhere. If heeded, it can save you a lot of needless headaches and frustration.
This third one came a few years ago from a blog post by Suzy Welch and ties in nicely with the first two. She wrote; When looking at an issue, ask yourself will this matter in 10 days? 10 months? 10 years? Depending on your answer, you can then gauge how much time, effort, and energy you want to spend dealing whatever it is.
Last, but not least, is: never compare yourself to anyone, only to the person you were yesterday. I can't remember where or when I picked up this last tip. Probably from a Twitter or Instagram post. Nevertheless, it's a solid piece of advice. For me, this last one is pretty much the key to happiness. Constantly comparing yourself to others is a sure-fire recipe for unhappiness.