I am going thru something similar. My cat died this morning, and looking back he was off his food and had gained weight - I thought one of the neighbors was feeding him, but I should have taken him in. I hope like you that he died in his sleep based on where I found him.
Our cats had rich happy lives and the love of their owner. I would focus on that and be gentle with yourself. Your cat forgives you. Please forgive yourself. Sending you a hug and condolences.