Hey. In agreement with everyone else; You are not the problem and do not ever let anyone convince you otherwise.
Your adoptive mother is very wrong to behave the way she does, it's fairly obvious, and is not doing a good job as a mother. When she made the choice to have a child (adopt you), she made the choice to undertake everything that comes along with being a mother; even all the difficulties, consequences, and responsibilities. It's a no-brainer.
Not caring for your child and refusing to give them the aid they need when they have an accident? That is awful.I'm so sorry to hear it.

It's good to hear that you love her for being your mom, but do not feel as though you have to love her as much as possible just because she is your mom- you would have every right to have conflicted opinions on her from what she has done to you.

To me, it kind of sounds like... I don't know, some form of mental abuse or manipulation, maybe? The fact that she very often makes you feel like you are worth nothing- making you feel that way at all, is just beyond me. That is not what a mother should do, and raises a big concern. It's really not healthy for the mind. And the way in which she occasionally acts nice in order to make you forgive her everything and keep her in good light again, well, think about it. Doesn't it sound kind of like some sort of manipulation and mental harm?

I really feel for you, and I honestly wish I could be there for you and give you a hug . I wish your mom would wake up and realize that what she is doing is wrong, and that she begin to treat you the way you deserve.

Btw, please don't feel as though I am trying to antagonize your mom as much as possible- I simply want to put forth what I think. I myself am a 15 year old (turning 16 in December) girl, and I really value my relationship with my mom- even when times are really difficult and we become distant, I love her to bits.
I hope my reply helped you in some way or another and that you feel better. If you ever feel as though you need to talk, know that you are not alone in this community, and feel free to post more or PM me if you'd like.