Tell them, at a quiet time, what you've posted here. That you want to understand yourself better so you can live the life you desire, and can be the best you can be!
I know you are 'only' 16 , yet you are nearly at adult age and moving toward that can be overwhelming and frightening. So on the one hand, you are 'only' 16, but on the other, you are 'nearly' an adult and you are thinking about important things and making a sound decision. Of course you want to understand yourself better, feel better, and feel free to be yourself and show your emotions. You want to be the best you that you can be!

They may not understand why they can't fulfill the therapist role for you.
They may feel threatened or judged as not good enough parents.
So let them know that this is about you and not about them.

If they are resistant to the idea, you could negotiate/request a trial period to see how it goes.

How do you imagine they will respond to your idea?