Can I perform Istikhara prayer to reassure my heart about whether or not I will be accepted for a job, knowing that I did not perform Istikhara before applying for the job?

Istikhara is prescribed when one intends to do a permissible act, and it is not a means to know the unseen. Knowing acceptance or rejection in the aforementioned job is a matter of the unseen, and knowledge of the unseen is exclusive to Allah alone. Allah the Almighty says: "Say: None in the heavens and earth knows the unseen except Allah," and He says: "But when he fell, the jinn realized that if they had known the unseen, they would not have remained in the humiliating torment." Therefore, Salat al-Istikhara is not prescribed for knowing acceptance or rejection in this job.