Hi faceman,
you won't deal with it, no one does. You are not expected to deal with it. Like your own mortality if you sit and think about it, personally, I know it's inevitable but that doesn't make me accept it easily and that's where my first comment comes from.
I've had cancer in my family four times, lost a parent to it, seen chemo, radio therapy, macmillan nurses, hospice, morphine, all the bits. I still know nothing, but your post is something I can't help but to reply to, to say what I saw in my experiences.
The first thing is not to lose you in all of this, it's the biggest shock you'll probably ever have. Get the news out there to friends because they will be such a help later on. You say you don't have anyone, you got PM's, you've got these posts. You have this forum, that's a few at least.
Second, time will seem awfully finite to everyone, you gauge that on how your mum feels. If she's not up for it, close it down. Her comfort is all that counts right now. Same with doctors, if meds or treatment are not covering it, get on the phone.
Third, don't go pointing inwards, you said all the bad things you did, well, look at it this way. Whatever you think you did bad, your mum won't be thinking that, ask her. That's why they are mums. That's what makes them special. That's why this hurts.
Fourth, know this one just now. You'll think about your mum alive or gone for the rest of your day's I can promise you that, these feelings will be very raw to begin with but time will make you cope better, in time you'll smile again. That "time heals" comment is true, it just has a sucky tag line. In time comes acceptance, somewhat. What makes you angry can be used, example, take up weights, learn something else, put yourself into hardship in the name of. If it hurts, hurt, show it, confide. If you think you'll need help, look now don't wait for it to happen. Make a GP appointment now.
And fifth, congratulate your mum on raising someone with savvy that when **** got real they expressed themselves and were inclusive instead of exclusive.
I've never used PM's on here, you have my ears if I can help you in any way at all.
My best wishes to you.