I also did my flying on the Seneca but then did my MCC on the 737-400 sim. I feel that like a lot of things, all you have to do is stay ahead of the game. Surely you felt the same way when you was going from a single to the Seneca.
Like everything, the task seems a lot harder until you have done it. Flying a 737 is just about anticipation, if you know what it's going to do then eventually you will find that it's not that much of a beast.