if you're not motivated because you have missing work, then cut your losses and just skip to the currently taught content because the longer you try to say "i'll get this all done" the longer you do nothing, thus making the missing work mound grow. it's a feedback loop
This is also excellent advice. You have to know how to cut your losses. Staying up to date is more important. Maybe keep that work in your back pocket if you want to boost your grade and your teacher will let you turn in ancient late work.
The caveat is that for certain classes, you need to know the previous content to understand the next chapter. Math classes are like this. If you fall behind, you might not be able to do the current work. In that case, and this is dependent on how much time you have, do as much work as you feel you have to do in order to understand it.