How can one differentiate between the fear of academic failure after praying Istikhara and the desire to withdraw, and this fear being a sign from the Istikhara itself or merely a whisper from the self and Satan?

Istikhara is recommended when embarking on a permissible matter whose correct course is unknown. It is a request for good from Allah Almighty and a delegation of the matter to Him. If a Muslim performs Istikhara and delegates his affair to Allah, then good lies in whatever Allah chooses thereafter, whether it is proceeding with the matter or abandoning it. Your withdrawal from the aforementioned study after performing Istikhara may be better for you.