What can I do about my feeling of resentment and difficulty accepting not undergoing surgery after praying Istikhara for it, knowing that if I pray Istikhara with the intention of not undergoing it, problems reappear?

The Istikharah prayer is legislated when a Muslim is in doubt about a matter. It is preferable to repeat it when in a state of indecision until one's heart is inclined towards one of the two options. To overcome indecision, it is advisable to seek consultation before performing Istikharah, such as consulting another doctor or changing hospitals, then repeating the tests and examinations, and after that, performing Istikharah to Allah. If one's heart is then inclined towards a matter and it becomes easy, then that is what Allah has chosen for them, as Shaykh al-Islam Ibn Taymiyyah said: "So if he performs Istikharah to Allah, then what his heart becomes inclined to and what becomes easy for him among matters, is what Allah has chosen for him."