Does the questioner's not continuing in the new job after performing Istikhara count as ingratitude for God's blessings, especially with his feeling that unemployment is a punishment from God?

You must entrust your affairs to Allah and rely upon Him in all your matters, for He alone is sufficient for you concerning what troubles you, in affirmation of His (the Most High) saying: "And whoever relies upon Allah - then He is sufficient for him." You must take the necessary means and diligently and assiduously search for suitable work, while relying upon Allah and being content with His decree. Do not let mere fear of illness prevent you from working, and benefit from past experiences to be positive and strive to achieve your well-being, putting in place a new plan for searching for work while taking the necessary means, relying upon Allah, and having a good opinion of Him.