Crazy as this sounds, one of my two Shih Tzu's is 17 and blind and deaf, has been for two years. I can't believe what I've learned from him. He navigates the house by himself, he goes out the doggy door, down four steps (during the day)and out to do his business. His determination to deal with his disabilities has such a positive effect on me.
He gives me that "I can" attitude. He is an inspiration to me. So, I am not sad, his life is a celebration, and I embrace each moment.
I too am an only child and used to entertaining myself, so when I am alone, I am never lonely. Perhaps you need to revisit hobbies that you have enjoyed in the past, or, find new ones. I grow orchids, feel so proud when they bloom, the beauty of their blooms to me, is breathtaking.
The bottom line, think outside of the box, try something new!