Bedsheet twisted into rope is underrated in my opinion.. I'm fairly certain I could've ended it if I stayed in the position, damn survival instinct. If you aren't passing out fairly quickly it's just not tight enough. Put your thumb on one side of your adams apple (or where it would be if you don't have one) and the other fingers on the other side, you will feel a pulse.. restricting these veins/arteries even with your fingers would make you pass out.. this is what your ligature needs to target, if this is done, your support doesn't snap and no one finds you, there should be nothing else to it. I'm no expert but its as simple as that from what I know, and obviously I'd rather you not do this but you seem fairly certain, and not being able to do this can be frustratingly agonizing and make everything worse, I understand. Good luck