Dear Tan, I've only just seen this now and want to say just how very sorry I am and I'm thinking of you.
I nearly lost Swampy when he was an outdoor cat, to cars and also getting himself shut in cellars. My late Mum told me I was taking years off my own life and to keep him in and get him a companion. About the only advice of her's I ever followed, but it was good advice. Swampy was unhappy for 6 months, but then became a relaxed, loving and actually happier cat. They do need more attention though and if you can bear to get Gizzy a 4 legged friend when you feel up to it, as long as they get on well, that may be the answer.
We had a terraced house and built a small cat run attached to the window and screened the other windows. Looked not so lovely, but b*gger the neighbours if the little loved ones are safe. x