Don't punish your daughter by terminating an apparently important relationship because you feel threatened by it. Instead, work on your self concept and your own relationship with your daughter. Don't expect it to be the same as she has with her therapist. Her therapist isn't her mother, and you aren't her therapist. Kids will share different things with different people in their life (don't we all really) depending on which audience feels safest, most interested, most relevant, etc. That is completely normal. No need to feel threatened by it.

My son sees a therapist (my therapist actually), and I value that time for him. That is HIS time and he deserves that safe place to unburthen whatever he wants. That doesn't mean my husband and I aren't "safe", but I'm a realist. I am absolutely certain there are things he would rather not share with his mom and dad at all. He needs that privacy, and we honor that.