How can a fiancée overcome her feeling of inability to conceive, and is there a specific supplication for that?

A Muslim should have a good opinion of Allah and attach his hopes to Him, for He is as His servant expects Him to be. He must ask Him and supplicate to Him for his needs, for Allah loves to be asked and is angry if He is not asked. A Muslim should not surrender to despair and despondency concerning Allah's mercy, for these are among the whispers of Satan. What you are suffering from are merely whispers and delusions, so turn away from them and turn to your Lord in all your affairs, including the matter of offspring. He is the one who bestows males and females upon whomever He wills and makes whomever He wills barren. Reflect upon the stories of the prophets in the Holy Quran, such as Abraham and Zechariah, peace be upon them, to realize Allah's ability to bestow offspring despite the apparent absence of means.