How do you decide a niche?

I start coach training in August. I am torn between working for TLCS or on my own. I’ve had a successful business but also know the experience and depth of train would be so valuable at the life coach school. Plus I like the idea of being a part. My thoughts on a niche seem to be coming from my life experiences. I’ve been an addictions counselor as well. Any thoughts on how to narrow down a niche would be great! Is it ok to draw from ones own life experiences, wisdom from those?