If you could write one message, leave one lesson for a past or younger self what would it be? What advice would you give or what kind of things would you warn against?
It can be anything in any way. Whatever you would think would best serve or serve at all.
You will expect something as corny as this even now from your future self I am sure. But here is the advice I can give you that might actually do some good.
You will love a lot but be loved little in return. Do not let this be as sad as it sounds however because it is only when you come to love yourself that rewards actually follow.
You will be smartest even when you feel stupid. You will be your most beautiful even when you feel ugly.
The friends you trust the most and have known longest will be the ones to turn their back on you the quickest. And the people you least expect to back you will.
You will think youy make a great many mistakes but you should learn quickly everything does happen for a reason. You will loose one job to get a better one. You will cast off one lover to know a sweeter one. You will loose friends you swore you would never loose and when you wake up and known you don't have to die without them you will understand self worth has little to do with others. This does not mean it will not hurt.
You WILL cry and people WILL tell you that you are weak for doing so. And you should tell them to leave you the heck alone because crying is human and there are times you will need to cry.
Everything else I could tell you only you can learn and taking my word for it or giving to much awya would ruin the lessons and I would not be who I am today.