Ok, here I go:
I'm happy...thank you for your existence and forgive me when I do (~~sometimes~~) my silly mischiefs / for my silly mischiefs I do at times
In the second part...she wants him to forgive her for the silly things she does, sometimes. This is romanian slang, kind of playfull and silly misbehaviour.
Muito bem, Selia. Mudaria apenas uma coisinha:
"Sou feliz... obrigado por existires (soa melhor) e perdoa-me pelas vezes que faço as minhas estúpidas travessuras."