You ARE a good mother. You went off your med so she could be healthy, went through ECT so that you wouldn't hurt her with meds, went out of your way to find a good family, and faced the truth about where you were at that point in your life. You wanted better for her, and put that first. There is nothing more responsible than that.
If I went through that, without the depression, I know I would be devastated.Talk to your docot about the Valium and see if you can't find something that doesn't make you so numb. I would expect giving up you little girl to hurt. In my experience this is completely normal when you lose someone you love so much Be nice to yourself!