I am going to offer some perspective, but first I want to say I understand your grief, I really do.
I've only known two cats that have died. The first was treated for 16 months for terminal heart disease, and I was very sad after he died. He was my girlfriend's cat, but I lived with him for seven years and we became close friends, and he was the first cat I had ever had as a pet/friend. It took about a year, but I "recovered" and realized that I had not done anything wrong -- I had in fact done everything I could to help him and his genetic condition was out of my control. Moreover, it occurred to me that I always knew he would one day die -- all living creatures die -- so in essence I had always known he would be gone at some point. One day I will die, you will die, everyone will die. That helped me put his death in perspective. He had lived 14 years, not the longest for a cat, but certainly getting into old age, and while I was sad that his last few weeks were not good ones, and his last 24 hours were rather bad, overall I couldn't be upset -- he had a long, wonderful, happy life. What more could I ask for?
On the other hand, I am responsible for the accidental/inadvertent death of a cat who was my best friend. We had such a bond of friendship, and I failed in my duty to take care of him -- in fact my actions lead to his death, and have lived nearly three years with that guilt. IT SUCKS. But, I know that there is nothing I can do to change the past or to bring him back. I have not forgiven myself and I probably never will; all I can do is try to make up for it -- and I did that by adopting another cat. It wasn't the easiest thing to do, but if I had never had my prior cat I definitely would have wanted him to have a good home elsewhere and not be killed in some so-called shelter, so I tried to do a good deed and save a life.
Where am I going with this? You didn't do anything wrong, and you gave your feline friend a long, wonderful, happy life, and he gave you 14 years of friendship. Certainly you'd want more time with him, as I wanted with my cats, but how much? You would still be sad after an extra day, or an extra week, or an extra month, even an extra year. You would be sad no matter how much longer he lived because when he eventually died you'd miss him. Does that mean he can live forever? You know that isn't possible. So, it's okay to feel sad, in fact it is quite natural, because your very close friend is gone. But, since you did nothing wrong, you cannot blame yourself, and you know for a fact that all animals have a life span. Thus, though sad, you accept what happened because you always knew it would happen. You will always feel a little bit of emptiness in your heart, but you will also always have great memories. This is the trade-off we accept when we decide to have a pet. Or, perhaps I'll end with a question, to which I think the answer will be "no": Would you prefer to have never had Charlie as your pet? Because that's the surest way to never experience the grief you're going through now. It isn't a trade-off I would ever make.