Why did God favor the righteous predecessors (al-salaf al-salih) and elevate their status, and does this mean that I have no share in this world and the Hereafter?

The problem is resolved once you know that sovereignty belongs to Allah Almighty, who disposes of it as He wills. He favors whom He wills among His servants, and He is not questioned about His actions. His favoring is subject to His knowledge and wisdom. Indeed, Allah has favored some prophets over others. It is narrated in Musnad Ahmad that Allah looked into the hearts of His servants and found the heart of Muhammad (peace be upon him) to be the best of the hearts of His servants, so He chose him for Himself. Then He looked into the hearts of His servants after the heart of Muhammad and found the hearts of his companions to be the best of the hearts of His servants, so He made them ministers to His Prophet. If you do not understand this, it is sufficient for you to be certain that Allah Almighty is not unjust to His servants, and that He is All-Knowing of who deserves guidance, and All-Wise in His actions, doing nothing without wisdom.