Looking at the Oxford Dictionary definitions for the two prepositions that you proposed:
in meaning 8 - As an integral part of (an activity)
for meaning 4 - Having (the thing mentioned) as a purpose or function
Both prepositions are correct: in would focus on their involvement in your studies, and for would focus on their purpose.
If you look at the entry for remind in the Cambridge dictionary
, you will see that one of the possible usages is (+ that), for example
Remind me that I have to go to the dentist at 3pm.
that is a relative pronoun here, and you can substitute other relative pronouns: what, who why, which, etc. so it's OK to say
...reminding me what to focus on...
It is not necessary to add of before what in this sentence. It's not wrong to include of, but it sounds a little strange.