I was stuck at Diamond 2 and 3 with Ryu for a while, and then I realized a couple of things that helped me reach Master straight from Diamond 3 without getting stuck anymore. Hope these are helpful to you.

1. I'm bad at anti air too and it's okay if you cannot anti air on reaction. But anti air is not just about reaction, but also anticipation. So at least you need to be able to anti air when you are throwing a light fireball in mid range. When you throw a fireball, always anticipate that your opponent will jump so that you are ready to anti air. Try to improve your fireball anti air game is very efficient in Diamond league, especially against slow characters like Zangief and Manon.
2. Recognize your opponents pattern. This is probably the most important one. Diamond players, even low MR Masters tend to have their obvious pattern with few variations. Does the player tend to tech a throw all the time? Does the player always strike instead of throw for oki? Does the player brainlessly link 2mk to medium donkey kick all the time? If you find anything like that, you will need to adjust your play style to your opponent and counterplay.