My best piece of advice I have is to always allow yourself to be happy because you only live once.. or YOLO! If you're unhappy then do something about it, only you can control your own happiness.
If something is making you unhappy then think about what you can do to change it and do it. If you're unhappy in your job, look for a new one or think of a career change. If you're unhappy with your haircut, book a hairdressers appointment. If you're not happy in your relationship, talk it through or find someone that does make you happy. If a friend is making you unhappy, talk to them about it because chances are they don't even realise. If it's something you can't change then try to think of the positives instead of the negative.
Everybody deserves happiness and it's up to you, yourself to make it happen.
I'd love to know, what's your best piece of advice? xxx