It's a common problem. Parent's sometimes see it, but are terrified by the possibility. It's a human reaction, hence people let that cough go for 6 months, or that lump, or that pain. I just had a very close friend die way too soon because he fell and hurt himself and was toughing it out and after 2 weeks he finally decided to call the doctor and go into the hospital to get it checked. That was the 29th of November, he went to bed that night sure he would go into the hospital on the 30th as scheduled and it would be nothing. Whatever it was killed him in his sleep that night.
So is what your mother did excusable, no, you're right, she should have done something. I'm sure there is a teacher or two out there, or some other folks in the same boat that might have seen it, but didn't want to be the one. Is it understandable? Yes. Is it forgivable? That's eventually up to you, but I'd hope so. Regardless, the only option you have now is to do the best you can to make up the difference and get to where you want to be. You owe it to yourself and spending time thinking about what should have or could have happened only delays it further.
Pretty uninspiring there I know, but its the unfortunate truth of the situation. Trust me, I've got a train wreck of a situation going that finally lead me to the discovery and it would have saved me a world of hurt, time and money, others too, but nothing I can do about it at this point, but do what I can now, with what I know now.