Good question. I will probably try to tell my younger self not to make the same mistakes I made, but it is true that these mistakes also made me grow. So, I will probably say to always try to be surrounded by good and quality people. And if someone is no longer helping you grow, or contaminating your atmosphere, just get far from that person.
I would also advise not to be worried about those low moments. We all have them, but if you believe in yourself, if you follow your dreams and your philosophy as hard as you can, you will always get compensated. And the most important… you will find happiness.
For me, happiness is to look back to the past and be able to smile. Being in the car alone and thinking about life with some nice music playing, and starting to smile, perhaps even throwing some tears out, without really understanding why. That probably means you are on the right path.