What lessons have you had to learn "the hard way"?

I learned that cheating and lying go hand in hand, and if you stay with a cheater it will probably happen again. I learned that you teach people how to treat you by what you accept.
I learned that you should definitely have at least 3 months of expenses saved up, because you never know when you may be out of a job. I learned I’ve been lucky in finding jobs easily in the past, but that may be because I’m more selective now.
I’ve learned to constructively disagree with people you care about.
I’ve learned not to drink when I’m upset.
I’ve learned that when it comes to your body and being healthy, if you don’t use it, you lose it. Exercise is essential.
I’ve learned that no good comes from gossiping, and that people are terrible at keeping secrets. It’s probably best not to have secrets, and if you do you shouldn’t share them.

Gosh, my list could go on forever. I’ll stop there, because I’m sure some of you have learned the same lessons as me. Elaborate on what the “hard way” was if you would like, or just share the ultimate lesson.

I’m all ears…