ranting I guess

Am
Let me start I'm a mommy to a beautiful 8 yr old little girl, a step son 8 as well, and soon to be newborn baby boy. 
My daughter has always been the most important thing in my life. She's my main priority I put her above everything. She has a lot of difficulties. Goes to a treatment school because of these. I know that I'm not perfect and I'm far from a great mommy. My daughter has a lot of difficulty mostly because of contradictions between the adults in her life. I could say no to something anything and my mother or/and brother would turn around and say yes. Everything is ok in there book. Tv always has what my daughter wants on. Dinner is changed if my daughter doesn't want what was planned. Which makes it difficult for me I'm the sole cook in the house. I do everything in the home. My daughter does not respect me due to my mother and brother always contradicting what I say. To my point. Today 6/5/17 my daughter had a dentist appointment for a root canal. She had up until 15 minutes before to change her mind and tell them she couldn't do it. I asked her once a day if she was ready and confident to go through with the task. This morning she decided she wanted other cavities filled instead. Which the dentist was fine with when I called and still fine with when we got there. Now 45 min has gone by and she refuses to open her mouth! I understand she's a child, she's scared. But she's also been through a filling before. At this point I was very upset my patience is very thin these days. I know I over reacted but I couldn't help it. I do NOT hit my child(ren) but when I am overly anxious, upset, and depressed I can yell. At this point I was at my breaking point. I told my daughter she will pay the large bill I will get, and I will never take her back again. That she won't go until her mouth hurts so bad all her teeth need to be removed. (I know very hurtful borderline abuse) I'm just so scared we've been told for almost 2 years that if the infections continue they will reach her brain and cause permanent damage and or death. I don't know how to help her. She doesn't listen to anyone unless it's in her cards. She does not respect anyone. She does not care about her health or hygiene. I can get her on a great routine and she be enjoying it then one night she wants to play dolls with gramma instead of shower and brush her teeth, and there we go it's out the window no matter what I say. I love my daughter more than anything she's my world. I can not go through her being so sick. I've already been there for 6 months when she was 1. They thought she had leukemia and it took so long to find out she really didn't. I was lost, miserable, suicidal more than ever, I had nothing to live for. If I lose her for something as simple as a filling I'd be devestated nothing can ever fill what she has made. I know I lost my temper I mean really lost it I was yelling and crying I had an anxiety attack, the thought of driving off a bridge crossed my mind. I told her all about how I felt when they thought she was really sick as a baby, I reminded her that the dentist and dr have told her it can go to the brain and will soon. I would never never never harm her or myself in front of her. I want her to remember how easy it was to get the last  one filled and do it to protect herself from harm since I can't jump in her body and do it. I want her to feel safe enough to let the dentist do there job. I wish she'd let them put her under but she won't. She won't go anywhere else. And now her dentist that she feels comfortable with is gone until fall. I just wish I could take all the fear, and bad things away, or really help her get past them. I feel like I'm no good for her. I know it's not true but when I have these moments I feel like I should have never become a mom.