I didn't have any trouble with the FMS. I got a copy of the software and played around with it in groundschool. Realworld flying is much harder and takes longer to learn than any system. And there are a lot of things you will not learn in the right seat of an airliner because

1) You have a babysiter
2) Airline systems and ops are so redundant that it is highly unusual to get in a situation where you are really pressured.