You gotta look after her as best you can mate. I imagine it's easy to give you a hard time as you're there and these other ******* aren't. I don't know you or her, but I bet you know what your Mum would do if the boot was on the other foot, she'd look out for you.
Doesn't sound like your fault at all and she probably knows that, but it'll be hard for her to admit that to herself, let alone you. I think it's irrelevant who is parent and who is child, you're all family and you need to stick together if you can. I imagine she will be finding it difficult (and probably embarrassing) leaning on you for financial support, parents never really expect things to go that way, but sometimes they can. I've got a little experience of this myself the last year, but while ever I'm working I'll do what I can.
However, there's no point in letting it go on as it is, you need to talk it out calmly, but sounds like a conversation about "living within your means" is necessary, it's not palatable and it will be painful and ultimately she'll already know that, but unless your financially set to keep picking up the pieces, something needs to change. Don't lose your rag, though I'm sure you'll be tested.
Good luck bud, it's not gonna be easy but sometimes you just gotta suck it up and do tough things. Life can swing round pretty quickly and you never know what's around the corner.