Well, you obviously care about your daughters
and that to me says you are a good mum. Far from failing!
It's tough, especially when PND is added to the mix, but you are far from a failure. You can only do what you can, remind yourself that your children are loved and cared for, get out as much as possible even if its just for a walk/drive to the park, take some time for yourself when the kids are asleep or with someone else. Focus on the small things, they make the world of difference.
Don't be too hard on yourself OP.