Really glad you've got plans with dd, and that you're feeling less detached. It's not true to say you're not doing her any good. You're there for her and she knows you love her - that security will be the most important thing for her and nothing can replace it. Don't be anxious about her...children really don't need a lot - some food, attention, love....she's fine I promise. It's natural to worry.
I think you have to really fight those urges to be alone. You don't have to give into them - you can fight them. You might want something, but that does not mean you have to do it. Put some safeguards in place - maybe plan something to do on the Saturday. You can change that decision and make it a positive one.
I think sometimes you have to realise that those feelings you're having that it 's selfish to be here are not real. You're feeling them because you're so low, and you'd know they weren't really true if you felt better. Look at your daughter smiling - she needs her mum to be around and not leave her. In that dark place I know you think it's best for her, but she would grow up not understanding why her mother didn't stay for her. I'm not trying to guilt trip you here, I'm just trying to be truthful. She needs you to be around when she comes home from school, when she has her first job, when she marries, when she has her own children.. She needs you to support her through all that. If you're not there, who knows where her life might go.
I also have a friend who's mother committed suicide when she was 8. Her mother undoubtedly felt a lot of the things you did - a feeling that her daughter would be better off without her, that she was too much of a burden. All my friend feels is grief and confusion, and I don't think she's ever understood how her mother could leave her. I went to university with her, and whilst both my parents were there at graduation, she only had one. Please please don't let this be your daughter.
The stress and hurt for other people is nothing compared to what it would feel like if you kill yourself. You wouldn't feel anything, but everyone else would be in terrible pain, no matter what you think. People are trying to help because they care about you - the only thing you can do to help them is to keep on going, day by day, staying alive.
Sorry this is so ridiculously long - hope you manage to get through it. But if it helps even a tiny bit, it's worth it.