Big hugs OP. Try to remember that your Mum would not want you to be suffering like this. She was in a place with professionals whose job it was to help her through the process medically. Your job was just to be with her and love her and you did that.
As a PP said there are threads out there, Reddit and so forth about people who "clinically died" and from what I've seen they report either nothingness or a great feeling of peace, I remember someone saying it's so much harder for the people who have to watch it than the person experiencing it. No they didn't absolutely die and they came back but the point is that they were not feeling great distress even with such bodily trauma.
Of course you can't have the absolute certainty of knowing her state of consciousness. But you can know you gave her all the love and support you had to give. Guilt is often something we are left with after a death and we always feel there was more we could have done. Somehow, to heal this, you'll need to find a way to let go. Not of her or your love for her but of clinging to the guilt. A way to put it down and let yourself breathe. And in this I'm sure you'd have your Mum's blessing.
Life and death are huge and frightening and we want so much to be in control, but we aren't. You weren't and couldn't have been in control of what was happening, the process took over and it is natural.
In some cultures there are rituals like chanting for the person who has passed, lighting candles or incense, and these can bring some comfort, don't know if anything like that would be meaningful to you. Or try talking to her and telling her how you feel, and try to make some kind of peaceful and loving connection with her or your memories of her? Forgive me if these suggestions aren't helpful.
All the best to you.