I heard that inner motivation (motivation from the inside, in other word, yourself) has more impact than outer motivation.
That was my problem too when I was preparing college entrance examination. My teacher told me that you can get a motivation by categorizing what you love, what you fear, and what you hate. Someone can get a motivation because he hates something. So, write it down! Repeat what you've written regularly before and after you're going to bed.
Side note: I would recommend James Clear's reading list for motivation [1].