Is it possible for a supplication to be difficult to fulfill, or for Allah to withhold His response to it due to the sins of the servant?

You must remove the feeling of despair and believe in the ability of Allah Almighty to answer your supplication, for nothing is impossible for Him. He says in the Hadith Qudsi: "I am as My servant thinks of Me, and I am with him when he invokes Me." Your supplication will either be answered directly, or evil will be averted from you because of it, or it will be stored for you as a reward in the Hereafter, for the Prophet, peace and blessings be upon him, said: "No Muslim supplicates with a supplication in which there is no sin or severance of kinship ties, but Allah will grant him one of three things: either He will hasten his supplication for him, or He will store it for him in the Hereafter, or He will avert from him an evil equivalent to it." The delay in answering may be due to the presence of an impediment or the lack of one of the conditions for supplication.