I would suggest testing the bag to see how quickly the feelings of suffocation and panic happen. It's so fast.
Realistically I think you would have to have someone put the bag on you after you were asleep.
I'm so sorry you're feeling so desperate. Unfortunately it takes time to find a method that's right for you, that has the least amount of suffering, that you can tolerate, that you have the resources for, that you won't potentially get rescued from or seriously harmed by and end up in the hospital. You'll see on a lot of the threads here that people test methods first, I've learned a lot from testing, or trying a method and being able to quickly abort if needed. It's important to know for yourself what you're trying to do and what you can tolerate. I recommend the methods megathread; see listed there the hanging mega thread, you can read how many times people test to find the carotid, work on building up tolerance, etc.
If it were easy to ctb when one is desperate, there'd be a lot less people on this site - and in the world - right now.
Good luck finding a method and managing your feelings while you search. Feeling empathy, sending compassion.