Please note that this is not "The hardest thing you've ever had to do". You can go through an incredibly difficult experience and learn very little from it. Difficulty does not imply quality or self-growth. I'm not saying that those don't have their place, but that place is not here.
This is for the lessons that have hurt the worst, or taken the longest, or simply been the most difficult for you personally.
For me the hardest lesson has been that I can't help everyone, and that I can't make myself responsible for everything. For a long time I tried to make everything work, and to fix everything, and to make everyone ok. I wear myself out trying to make sure that everything is just so, and the everybody's happy. And if someone isn't doing their job, I can feel like it's my responsibility to pick up the slack. I try too hard to do everything, and be everything to everybody.
I know it doesn't work, but I'm still dealing with the fact that there are some things that I can't make better, and some people who I can't fix.
What's been the hardest lesson for you to learn? Are you still trying to deal with the consequences of that lesson?