I think for me what worked was being busy. I needed to be out all the time coz I think I felt lonely. If people came to my house I would've been ok at home, but they didn't, so I was never home.
I was always off shopping, or visiting someone.
Sleep deprivation is one thing, but add lonliness & lack of adult conversation to it & it gets hard.
Being there is a good start. Do you go to a palygroup? Having other mums around might help.
Just going for walks. Coffee every couple of days. Help her get out of the house. Or offer her excuses to anyway.