Not OP but while I aced my class that used Sipser's book, nevertheless I still lacked the mathematical comfort/maturity to do Arora and Barak. The first class started talking about "random functions" and I had no background for that at the time. I don't know if some kind of advanced discrete math class would have helped fill in the gaps at the time, at least for myself. Or maybe a remedial discrete math class, I was mainly an engineer not originally intending to be a CS major.