For me it was getting used to the sim and the procedures. All the training I had been through in the past was accomplished in the actual aircraft. The sim was just new to me so there was a bit of a learning curve. Also, all my previous experience was part 91 with little SOPs. The 121 environment is completely different.

The instructors do a good job of picking every little thing out that you are doing wrong, but for good reason. This does get tiring though. I really believe the key to it was showing up ready to learn and studying your butt off.