Is it considered a mistake to believe with absolute certainty that a specific prayer will be answered, leading to the rejection of suitors, based on Allah's saying: "Call upon Me; I will respond to you"?

You did well by repenting, and there is no harm in your prayer that Allah makes him your husband. However, your trust in Allah answering your prayer does not contradict your acceptance of marriage to someone whose religiosity and character you approve of. The answer to a prayer is not necessarily the fulfillment of the requested specific thing; rather, it might be the hastening of the supplication, or its storage for the Hereafter, or the warding off of evil similar to it. We advise you to accept whoever proposes to you from among the suitable men. If the young man is righteous, there is no harm in you asking him to propose to you, while being careful not to return to the forbidden relationship.