You'll find that some of the causes you stood up for were corrupt, that you obsessed over minutia while ignoring what really mattered. You will do some of the things you used to harshly judge others for, and pray that the world is more lenient now than you were back then.
Oh, And You Will Sometimes Curse The Person You Are Now
I assume some of you do this already, wishing you could go back in time and kick your own ass. Not just for what you did ("Why in the hell did you smoke that first cigarette? Did you think it would make you look cool???"), but what you failed to do -- the plans you didn't make, the crises you didn't prepare for. But it's hard to grasp that Future You will feel the exact same way. ("Why didn't you quit smoking while your lungs were still healthy enough to recover? What did you think would happen???")
And having been through a few decades now, I can tell you that yeah, that never changes. The personality traits you like the most about yourself now will make Future You cringe. A whole lot of your present bravery will turn out to be youthful ignorance of consequences. You will someday groan at the thought of what 2020 you took for granted, all the chances you wasted.
I mean, you will if you're lucky. Improving a thing means the past version now sucks in comparison. The worst outcome is a Future You who mourns the awesome person you once were. If you play your cards right, if you try to fix one tiny thing every day, your reward will be that Future You will scramble to erase all evidence that Current You ever existed.