galinaqt.. if YOU think its a powerstruggle and are sure its not medical then a psycogogist isnt going to help your daughter.. if its a powerstruggle with you then you need to change what you are doing for her to change what she is doing. you have to stand up and be the parent and show her that you mean what you say, not just in this area but all areas. she needs to knwo that you are the adult and she is the child and that basically what you say goes.. it sounds like she walks all over you and until you do something to fix that, dont expect to get the results you are wanting. in my opinion sending her to a psychologist is you telling her that there is something rong with her when in fact, if its a power struggle, then its not her problem its yours. Everyone likes to be in control, kids are no exception but as adults its our job to clarify the boundrys, you cant always have them do what they want or they will think they own you! im not saying never let her get her own way, kids need a bit of that too but she has to respect what you say. you have to win some of the battles, you win the big stuff and you give her the not so important ones.. it sounds like (from all ur posts) that your little girl runs your household instead of you and your husband. if you let that continue then you are going to have more problems than just this!
I sincerly hope i havent offended you here but i dont think its your daughters fault who does not need a psychologist but instead needs some clear boundrys set by you in other areas and then the toilet training will fall into place...