Amylou, please call someone, a friend, a relative, your mum, a neighbour, your doctor, someone. You must not think like this. Cleo and the kittens need you. I do not believe you crushed kitten in your sleep, that is extremely rare, I have never heard of it happening especially on a couch where the little one would be pushed into the pillow, not against a hard surface, and would be able to wriggle to breathe. But even if this did happen, it is a terrible accident, no one will ever blame you for it as much as you blame yourself. It happens in nature too, the reason why sows are put into sow crates when they have litters (although I think these are very cruel) is so that they do not accidentally crush their piglets. These terrible accidents happen. You are not the first person to contact this website in agony because you believe you have accidentally killed your kitten in some way, and sadly, you will not be the last.
A post-mortem on kitten might ease your mind. But right now I am most worried about you. Please reach out to someone. You need help and counselling right now to deal with the grief, please do not think of harming yourself in any way. Cleo and the other kittens need you, what will happen to them if you are not there to give them a home and buy food for Cleo to feed her babies and look after them? Call someone, please.