Sometimes pre teens and teens can be impulsive with what they say. I have two myself and sometimes it is shocking what comes out of their mouths. They do not mean harm, they just don't know how to balance their hormones, adult feelings and child behaviors. They also don't have the ability to think things through like adults do.
The best thing to do if you feel hurt by something your daughter says is walk away for a few minutes. Get your bearings then return and let her know you feel hurt by whatever it is she said. Tell her calmly and gently. Kids usually feel bad once you let them know how you feel. It can turn out to be a way to bond when you talk about how you feel with your daughter.
I am a little surprised your therapist did not want to see your collage. I would think that it would help her help you with how you feel. Although she may have originally meant for you to do the collage at home for yourself, sharing it with her could only help. Maybe it was just a misunderstanding on her part.
I hope she responds well to your letter. You have given her a heads up about it so that should help. But be prepared for however she may react. What if she does react with indifference? How will you feel? What if she wants to discuss it at length? Think through all the possibilities so you are prepared. That will help you.
Let me know how it goes,