I am in dilemma about my child`s custody due to poor mental health. is it due to hitting ?

Don't understand: I'm sorry, but do you mean you're concerned about your child's poor mental health, or yours? Who is doing the hitting -- your child, you, or someone else? Are you being hit by someone? Please seek out a therapist, or speak with your physician about these things -- they are serious and deserve medical (and possibly legal and/or child protective services) attention.

Depends: There is research indicating that child abuse/neglect is correlated with poor mental health outcomes later in life.