Is the Istikhara prayer valid when feeling regret for a decision made after performing Istikhara?

Istikhara should be performed before the contract. The lease contract, once entered into, is binding and cannot be rescinded except for a legally valid reason. Psychological discomfort is not a valid excuse. If both parties agree to the rescission, then there is no harm. However, if the rescission is initiated by one party without the consent of the other, then the full rent for the remaining period of the contract must be paid, because a lease is a binding contract. A mere psychological feeling towards the worker is not an excuse that permits rescinding the contract without her consent.

As for Istikhara, it is not necessary for the person performing it to see a dream. Rather, whatever happens after Istikhara, whether it is proceeding with something or refraining from it, is what Allah has chosen for the individual.