I dont know if you will feel the same Jeremy, but I often think that getting back on your horse when you've had a bad fall is the best way of dealing with things.
We lost Mac, our last cat, to a road traffic accident last May. It was the day before his first birthday, and it followed the loss of our 20 year old Cat Paddy, and then Tess, our beloved Beardie cross dog. We were absolutely devastated, and I really didn't anticipate getting another cat within any measurable space of time. My OH was equally affected, and as you say - each of us was bound up in our own pain, so it was difficult to offer comfort to each other, although we did try.
Surprisingly, my OH decided within days he needed to take on another cat, as his only way of being able to cope. I wasn't at all sure, and felt somehow it was being disloyal, but in this my OH was absolutely 100% right. It didn't diminish the sense of loss, or the pain we felt at losing Mac, but having another avenue of focus gave us something of a distraction. And such is the way with cats that having Ross helped to give us a reason for getting up - another life to take care of. Very quickly he became part of our family and wound his way deftly into both our hearts.
We still think of Mac, and of Paddy and of Tess and all of the beautiful animal companions who've shared our lives, but less painfully, and we remember the good bits. We look at the photographs and we feel the love as strongly, and it's all been down to Ross, because he gave us back a purpose, and stopped us from looking inwards too much.
Allow the thought to grow on you. Having another cat wouldn't be disloyal to Jonah, if you decided that's what you'd like - cats dont judge us like that, they're far too sensible. What it might do, is give you some joy back, and something to think of in a pleaasurable way - the excitement of getting to know another friend, and showing him or her how much love there is in your home.
After 20 years of having a fully grown cat, we adopted a kitten, and then when we lost Mac, we took the plunge and adopted another kitten, and seeing him come into young adulthood now has been tremendously pleasurable. And when he was an out and out kitten he was so funny. You see their character develop right in front of your eyes, and little personality traits developing into bigger ones - I'd forgotten about those particular delights. What's not to love?