I technically went thru destoryer 4 times.
twice last year, twice again this year.
If you worry about memorizing the problem. after completing it. put it away for like a 1 week or something. use other practice materials, then come back to it. start with a blank notebook or something so you can't look at your old work.
at the very least I suggest you should spend time just reviewing the problems that you worked out. know how what you did and why you did even if you don't want/have time to re-do them completely.
like for ochem there are plenty of multi-step synthesis problem, you should go over each step and know understand what the reagent used is doing.