How can one overcome the devil's whispers that allude to polytheism (shirk) and attachment to Allah, especially after repenting from an illicit relationship and marrying, while maintaining prayer?

Our advice to you is to turn away from misgivings (waswas) and not pay attention to them. Strive to repel and rid yourself of them, knowing that you are on the religion of Islam. Therefore, do not pay attention to what Satan casts into your heart. To clarify the criterion for love that is considered shirk, please refer to Fatwa No. 105179.