Thanks for the replies. Sorry I didn't answer immediately, I started a new thread in the mental health section and didn't expect people to carry on answering.
Wannabeglam, she has a mother she's close to, but I can't enlist her help. They both share the view that depression is something you work through on your own, and both think counselling doesn't work and anti-depressants are more hindrance than help. Her mum has spoken to me and is aware that she's got a problem, but is taking the route of babying her. She's decided she's going to start visiting more often and helping take the load off.
Sandychick, thanks for the reply. It's valuable to hear about what helped others. I'm hoping as soon as I get her to accept there's a problem and that people can help her, she'll have plenty of resources to help her recovery. It's just getting her to accept it that's the issue.
FunkyCherry, she does post on here sometimes, but I don't know if she would feel comfortable posting about this. She doesn't even like talking to me about it, but she says things like: "I just want you to listen, I don't want your help". I can't help but instinctively try and give advice or suggestions, and that irritates her so she stops wanting to talk... :(
I always expected she would hand the baby over when I got home, but she seems to completely "switch off" from parenting when I'm around, sometimes she'll even say "I'm off duty now". At weekends I don't think she feeds a single nappy or bottle, and she scrunches her face when I ask her to hold the baby when I go to the toilet or make a phone call. My mum noticed it and mde a few mean comments about it, implying I shouldn't be doing all the parenting on my own. She seems really worried about ME, and wants me to start forcing her to help more in evenings and weekends, but I don't want to make her feel more pressured because I think she just needs one little push and she will leave. Not because she doesn't love us, but because she wishes she wasn't a parent anymore.