The more crimes you commit (or are convicted of committing) the greater the possibility of jail. It is not "double jeopardy" to be separately tried, convicted and sentenced for separate criminal acts.

If he wishes to take the cases to trial and argue that he's not guilty he can do so, but he should hire a lawyer to help him. Your boyfriend's lawyer can try to utilize the letter you wrote, if he deems it to have any value. I doubt that the court will allow much exploration of your mental health history - that's not what caused you to have physical injuries from the altercation.

You are expected to testify truthfully. If you lied to the police or intend to perjure yourself, consult a lawyer of your own.