Dealing with Dementia Triggers

I told her via text so she wouldn't see me crying about it my brain is huge

I know it's hard when it feels like you're someone's only resource, but you're not a therapist (I'm assuming), and even if you were, you wouldn't legally or ethically be allowed to be your mom's therapist. That's because you are too close to the situation to be a source of objective support. I know you have good intentions, but when this is so distressing for you, you aren't ultimately doing yourself OR your mom much good by continuing to subject yourself to this. If you don't want to draw a hard line and tell your mom you can't talk about this at all, you can set up some boundaries that may make it easier to cope. Asking her to check in with you before she starts talking about her day; asking if she's able to limit her time to maybe 10 minutes before switching topics; these are reasonable limits. It may take some of the pressure off if you know you have some control. And if you need to not hear about this topic at all, that's a completely reasonable thing to ask. You don't need to understand why it triggers you, and even though it may upset your mom, you aren't responsible for her feelings. There are therapists, and support resources for caregivers, that she can look into. You'll still be there for love and support in other aspects of her life. You aren't a bad child because you can't provide everything she needs- that isn't your job.