If you imagine the implied definite articles
The creation and the deletion of (...) accounts
then the need for 'are', or even 'are both', becomes a bit clearer.
But to solve the problem you describe as "and" here really means "or" it might be better to re-order the sentence.
Active Directory accounts are created or deleted automatically
during the ...
As 'thoughts' are plural, so subject-verb agreement tells us the correct form of the verb is determine. Yes, you made the right call.
Note, 'which' (relative pronoun) represents 'your thoughts' in the sentence.
This negative attitude reflects your thoughts which determine your success or failure.