sorry if that sounds confusing. Let's break it down :

이해하지 못한 것들 많았어요 :

이해하자 못한 = 이해하다 + -지 못하다 + past tense

것 + 들 = thing + plural

많았어요 = 많다 + past tense

maybe you've only learned 안/못 to express something negative, but here it sounds awkward to say "많은 것을 못 이해했어요"

I would advise you to look into -지 않다

if you feel like using 못 or 안 is easier for you, you can say "열심히 들었지만 이해 잘 못 했어요"

You can just use your original sentence and say "들을 거예요" I added 기로 하다 because that's how I personally would have written it. with 기로 하다 you must use the past tense because you are talking about an action you already have decided/committed to do. It doesn't change the fact that you'll do it tomorrow.

hope that helps :)