To be a writer, open your heart. Keep it open. You can't be a writer with a closed heart.
Be happy. Decide to be happy. If you want to be happy, be happy! No one cares if you're happy or not, so why wait for permission? And did it really matter if you had been deeply unhappy in your past? Who but you remembered that?
Childhood is supposed to be happy, and if you can't remember yours with any happiness, what hope have you later, when life starts handing you fresh grief?