Is total testosterone of 189 ng/dl in the "normal" range for a 15 year old male? Or is it low?

Not just T... Multiple factors make the ways you're, not just t-level alone. Your situation requires a comprehensive evaluation to look into your growing pattern, lifestyle, possibility of overindulgence and obsession, the size and consistency of testes, current appearance o f general manhood, etc. It's not adequate to focus on one t-level alone. So, find doctor with not just knowledge but also common sense.