I need help

I just finished playing the game,and after the credits the game loads me into the main menu, and after that I can’t continue to play the game normally. I saw some videos on YouTube, and what’s supposed to happen is that the game loads to a previous save before the last mission, but that doesn’t happen to me. If somebody had the same problem and figured out how to solve it,please help me i didn’t complete every side mission and I want to still play the game