Hi OP.
I lost my mum in a v similar way two years ago.
She was on a syringe driver and I knew the end was coming, but I had assumed it would be peaceful.
She started choking and she looked terrified. It was like she had something stuck in her airway. I ran to get a nurse, but the choking continued and she died (from what looked like lack of air). She died looking terrified.
She was aware. I know this because as she was choking I asked her two questions and she nodded and shook her head appropriately in response. The nurses tried to tell me she wasn't aware to comfort me I guess, but she was and people who said she wasn't just annoyed me because I thought "how do you know? I asked her questions and she responded appropriately so she was aware".. However, one of the palliative care nurses explained that although she was perhaps aware, she wasn't in pain due to the morphine etc... and her awareness wasn't the same as you or me. Because of the high level of drugs she was on, it was more as if she'd had 50 gins. So yes, she could answer my questions and knew she couldnt breathe, but was still very out of it. Like being on a night out, off your head, but multiplied 5 times. This bought me some comfort.
What also helped was knowing this way of dying is normal. Some of the nurses said they'd seen people die like this a lot. It's normal and there's nothing you could do. I always thought death would be peaceful. We need to talk about it more in society and acknowledge and let it be known death can look frightening etc.. so we are more prepared.
I had the image of my mum dying and looking terrified, stuck on my head for a long time. I still think about it every day but the image no longer has the potency it did and I'm able to move on.
It's actually helped me to read your post today, knowing I'm not alone.
I also found the Mumsnet bereavement board helpful and I read a lot of articles on line about people who died briefly during surgery or whatever, and came back to life and were able to recall their experiences, and that was helpful as all the stories were peaceful and positive.
Look after yourself. You have done nothing wrong. Like me, you were there with your mum. That's wonderful and she'll have felt comfort in you being there.
💐💐