I think that sometimes the kind of people who are inward-looking, driven to introspection and prone to conditions like dissociation tend towards carrying too much responsibility, guilt or blame for things that in reality are outside of their control. Whether it comes from ourselves, an escalation of difficult circumstances or other people shaming us, this kind of self-reproach can become toxic and crippling if you give it head space.
I'm very sorry about your grandma, but it's not your fault. There was really nothing you could do about it, and it was outside of your control. You are only one person and can only do so much, and you're not responsible for everything that happens. The fact you have DPDR shows you had already been feeling overwhelmed so may have been giving too much already, rather than not enough, and needed more support yourself. I'm sure she wouldn't blame you, and only want for you to be healthy and happy.