I am so sorry about Monkey. Dexter may just need some more time. We had to have our cat Felix put to sleep in April. His brother, Oscar, showed some of the same behaviors that Dexter has. It took Oscar a long time to adjust, but we, like you, just gave him extra time and attention and he is better now.
And, I know where you are with not wanting another cat. When we lost Sweetums, many years ago - I swore that I wouldn't get another cat - it was just so hard to let her go and I was very sad for a very long time That lasted three years, before we got Oscar and Felix as kittens. So, you will know if it is time to get another cat or to not get another cat. In the meantime, just give Dexter lots of love and attention.