Mothers have definitely taken the roll as the wisest humans ever. They've been through the worst and have learned the most valuable lessons. Now going through this time of my life and experiencing things first hand, I can definitely speak for my mother for some things.
If I were my mother I'd tell myself...
It's going to be okay:
There's never a need to panic because I've learned that it's probably not going to matter in another couple months. If I were my mother I'd tell myself to chill, not be so dramatic, and tackle my situations head on, because chances are I'm probably reacting way worse than I need to be.
Blaze your own trail:
When I was a kid, I was far from similar to any other little girl. The barbies I had, sat dusty on my shelves, whereas my spy gear was being put to more use than anything that had the color pink on it. If I were a mother I would tell myself that was completely okay, and being like everyone else is no fun. Never follow the beaten path.
Everything happens for a reason:
We've all heard this one before, but it's true. If you look back on things a few weeks from when it happened, theres usually a explanation for what had happened. The little voice in your head says, "Oh, *insert event here* wouldn't have happened if *Insert event here* never happened. If I were my mother I'd tell myself to open my mind up to negative events in my life and take it as a learning experience that helped create something great in the long run.
Don't ever settle for anything less than what you want, whether it be your relationships, your career, or the dinner you want that night. If I were my mother I'd tell myself to always strive for the best and never settle for anything less than that.
Don't take it so seriously:
If there's one thing I've learned over the years, it's to not take it so seriously. If I were my mother I'd tell myself that life is short. Do things you wouldn't normally do. Take the chance even if you're worried about what might happen and learn from the mistakes you make, so you can make a different mistake the next time.