Why has the worship of performing Qiyam al-Layl and reciting long surahs during Witr become difficult or impossible in our time, and did Allah single out only the Companions and Prophets for it?

What you mentioned about the absence of those who perform acts of worship like Qiyam al-Layl (night prayer) is an exaggeration. Goodness remains in the Ummah (Muslim community) and such people exist in every era. You will be rewarded for the night prayers you perform, and your consistency in this amount is good; you can gradually increase it. The best night prayer is that of David (peace be upon him): he would sleep half the night, pray for a third of it, and sleep for a sixth. Achieving aspirations requires diligent effort and perseverance. You should also supplicate and read the biographies of prominent and noble figures.