i have 50/50 legal custody.i am wondering if i have legal rights to have her stop seeing her current therapist and transfer to another one

You do not have any greater right than the other parent, so the change is not yours to unilaterally make.  Do you also have 50/50 physical?  going to a doctor is a physical custody issue.  A course of treatment would seem to be a legal custody issue.  You need to work this out with the other parent.