Ultimately it doesn't matter why she's doing this, or even if she accepts responsibility for it.
You know she did it, you have the proof you need to make the only decision there is to make.
Happiness doesn't elude you, it's just that you don't demand it. You settled for someone SO terribly awful because, for whatever reason, you think that's what you're worth.
Being on your own is better than being in a relationship where your DD is treated badly, and you are disrespected.
I know how hard this is. I've done it myself.
Can you look at this for a second with distance, as if this were someone else?
You can see what has to happen right?
Ok then, remove all feelings. Put them to one side, because most of those feelings are based on fear, lack of self esteem and perhaps even guilt or selfishness.
Now end it.
You can come to terms with the feelings of loss later. We can help you with that, trust us, we do this day in, day out! :)
You have to make the treatment of your DD, of you a total dealbreaker. She needs to go.
Sooner rather than later.
What you tell people/your DD is that your DD is the most important person in your life, and having someone who isn't prepared to treat her with love, kindness, warmth and respect is not acceptable. Ever.
You can do this. Do it now. This week. Every day spent with someone who makes your DD feel bad is a day more your DD will need help to recover from.