What should I do about the days I stopped memorizing the Quran? Do I have to make them up, or is there an expiation for them, and what is this expiation?

The Shafi'i madhhab (school of thought) regarding someone who vowed to memorize a specific amount of the Qur'an and then failed to do so, is that they must make up what they missed. This is the correct view. If a duty is missed, it must be made up. Even if they did not have a specific intention for the exact amount they vowed, their obligation is discharged by memorizing anything that can be considered a "portion" (wird), even if it is small.