Awww OP. It's telling that despite the physical abuse, what you most remember was this day when your mother left you when you needed or wanted her.
I think that's a manifestation of feeling emotionally abandoned. It's all well and good for people to say - I was also left at 13yrs old when I was poorly but it's the stuff around that which for you was different. Perhaps these other posters felt inherently loved in a way that you didn't, which meant it wasn't an emotional thing for them to be left alone while unwell. Perhaps their mothers fussed all over them before leaving them, maybe they left a drink and medicine at their bedside table, maybe these mothers showed how bad they felt having to go, maybe they called home at lunch time to check how they were doing but maybe the way your mother left you was just so unfeeling and uncaring.
I think we can all understand that. I've been stood in the doorway feeling bad about leaving my grown ass hairy husband at home with a cold - fetching him medicines, and leaving food and telling him to 'text if he needs anything, I can always come back and to make sure he phones the doctor' etc etc.
All of those things (or the absence of which) can play into how this specific instance made you feel.
I'm guessing overall it's part of a much bigger picture. Because if your Mum was someone who was there for you the majority of the time, then in this instance you probably would be more inclined to give her a pass on it and be more sort of 'I can't believe you did that Mum' a bit indignantly, perhaps even jokingly now years later. I'm guessing that your overall feelings are one of not being cared for more generally which makes this example hard to get past and for good reason. What I'm saying is, this example for you fits into a specific context of feeling uncared for which might not be the same for others.xx