What is the meaning of the change in the results of Istikhara prayer concerning marriage, and can supplication change destiny and unite me with the one I love?

What God will ordain in the future is among the unseen matters known only to Him, and Istikhara (prayer for guidance) was legislated for the purpose of intending a permissible matter, not to unveil the unseen. Therefore, your first Istikhara prayer was appropriate, and what you felt comfortable with afterward is good, God willing, so continue with it. As for repeating the Istikhara, there is no need for it as long as you have found comfort, and what seems like problems to you is from Satan, so pay no attention to it. To ensure the continuity of the family, adhere to rights, overlook mistakes, cooperate in righteousness and piety, and ask God for well-being. You can supplicate with any suitable prayer such as: "O Allah, make good our end in all affairs."