Do the doubts one feels concerning the supplication for marriage being answered, after repentance and reliance on Allah, indicate a satanic whisper or a lack of sincerity in one's repentance?

A true repentance erases the sin. It involves abstaining from the sin, regretting having committed it, resolving not to return to it, and restoring the rights of people or seeking their forgiveness. One must be wary of the whispers of Satan and despair, and strive in supplication without haste for an answer, for supplication is answered as long as the servant does not become impatient and say: "I supplicated, but I was not answered." Reliance upon Allah is one of the greatest acts of worship, stemming from trust in Allah, which in turn stems from knowing His names, attributes, and actions, leading to loving Him, glorifying Him, and trusting Him. Supplication is beneficial to the servant in every situation, for there is no Muslim who supplicates with a supplication that contains no sin or breaking of kinship ties, but Allah will grant him one of three things: either his supplication will be answered swiftly, or it will be stored up for him in the Hereafter, or an equivalent amount of evil will be averted from him.