Things that you used to lose sleep over or worry about, you now find to be so trivial and lame. You can't roll your eyes and discount others as they experience those same things and react how you used to react. You tell them that everything will be fine or this too shall pass, but nobody believes you no matter how old or wise you are perceived to be by others.
When I was a kid, my mom used to call me "Nervous Norman". I used to worry about a lot of things and most of them were things that were beyond my control. My stomach would get tied up in knots and I would pace the floor worrying about things as if they were the end all be all. I say "things" because here I am in my late 30s and I can't recall exactly what those things were! That's the funny thing about things.
Most of the things probably revolved around moments that I signed myself up for. Things that I brought upon myself and events that I could have easily avoided had I just not volunteered or competed. Girls I decided to like or be in love with. Sports I decided to play and competitions I decided to enter. All just things that caused self inflicted stress and anticipation.
As you get older, you sign up for things that are a part or an extension of your career or charitable things because you want to be a good person and you assume you have the time and energy. You coach a little league team, Girl Scouts, join PTA, the company softball team, etc. all things that take time and eventually add stress to your life. Hell, blogging is voluntary and it wasn't till I turned it into a book did it become anything more than just a thing I did. Turning it into something made it stressful. Screwing around and writing my thoughts for the internet to read, not so much.
Those things you do mean something. They add up to bigger things. Things that open up doors of opportunity and experience. Things that allow you to be influenced or give you a venue to influence others. These things challenge you and often push you outside of your comfort zone. These things create scars of experience and make it easier to get back up, having been influenced by something that eventually makes you better.
Do things. Do more things. Do the stuff that makes you sick to your stomach. Things that scare the shit out of you. Things that start out as impossible or ridiculous, but result in being the most rewarding. When you look back at all the things you have done in your life, it's pretty amazing. (I wrote a book. I still cant believe I wrote a book. It's not the best book or the most perfect book, but I wrote it). Do things that make you pinch yourself or that the 12 year old version of you would never believe. Prove your younger self wrong and do things that would amaze your exes and make them hate that they ever dumped you.
Do some things. Lots of things. Pick some things and do them. Getting started is stressful, but the open doors and experience that comes with these things is pretty awesome.
Even the dumb shit things.