Snarf, sorry you're feeling so low.
FWIW I don't think it helps to try and think 'why us' - you can't make sense of it, it happens to the nicest families and it can really rock your sense of justice and fairness. When I hear people say "what goes around comes around" etc like there is some kind of big plan where we all get what we deserve, it makes me cross.
None of us deserve this stuff, it's not a punishment or part of a greater plan. I know two families who have lost a child at around 5 years old. There's no 'why' that can explain that. Life is shit sometimes and how good or bad we are has no bearing on the suffering we have to endure.
Sometimes you just have to wallow in the sadness and allow yourself to feel utterly bereft, because that's what the situation calls for.
When you lose your parents life is never the same again, you can end up feeling a bit rudderless, when you're now the oldest generation of your family it's a scary thing. However, you will adjust and you will carry on her legacy as a mum in the way you behave and treat your own children. She will always be with you.
I honestly think this part is the hardest though. You do so much grieving before it's even happened, but you can't fully start to accept and mourn yet as you're in limbo. Then once the inevitable does happen it's almost a relief that the thing that's been hanging over you, threatening you is now over. I don't want to be disrespectful and I hope you take this in the spirit it's intended. I don't want to belittle your experience, but 10 years on and having several friends who have also lost at least one parent, I just want to say that it does get better, you will cope, you will still miss your parents at times, but you almost have to 'grow up' and start to rely on yourself.