I think it's good that I've been working on my mental health lately.
Wow that's great, why have you been working on your mental health?
It was time to start focusing on my wellness.
Working on your mental health is good but in order to do so you should reach out and get help and support from others. Keeping your illness to yourself is not an effective strategy and could very likely make your mental health worse. It would be a good idea to start regular sessions with a therapist to help you work effectively on your mental health.
I don't want my friends to know about my mental illness. I am a very private person.
I know you want to keep this private, but you shouldn't be embarrassed of it. Tell other people so that they can help you and understand your mental health struggles.