Its a very beautiful story, congratulations on your effort and patient, well done, it only shows that you are beautiful soul and you can really look after other souls....there was a reason you found that kitten or rather kitten found you, you might not know it now but one day you will realize it.....its a beautiful feeling when you do and you know why....
I went nearly same experience as you but my older cat is 13 i think, we got new kitten after the old cats best friend passed away so she was very stressed by that situation, it was after about 6 months we got kitten, still it was stressful for all of us, old cat, kitten and me and my wife. It took a lot of time and our effort but eventually they do get along. 2 years later they do lick each other.....and it was also beneficial for older cat too, she is a bit more active now, sometimes even initiate the play by herself....
Cats needs time, they have their own routine and dont like to break it....
Enjoy your time together, share all your love for them, you will get it back with a lot of bonuses....they do give unconditional love and we should appreciate it.....
Peace and Love!
In Lak'ech!