VORTIME,
formulaben is so right. Unlike the airline training setup, GA aircraft training organisations are geared towards more experienced people and don't yet cater that well for people who haven't been the seneca>navajo>kingair route (in time they will). They're going to assume alot about your instrument flying experience and possibly a bit about your multi-crew experience. Try and do things which will close that gap, maybe some Pilot Assist work if you can get it, practice departure and arrival briefs, and as someone suggested, even some 1-2-1 with the mighty MS FlightSim practising holding and procedural approaches.
Also, just look at the length of the courses: I know they aren't anything like the same a/c, but typically 5 weeks for the 737, 12 days for the Bravo. You can't do anything about this so, as always, preparation is everything: GET THE MANUALS and get some back seat rides if you can.
Critically, if you're not happy with something in the sim, be it the instruction or a particular hurdle you're struggling to get over, SPEAK UP and ask for help, you should get it. Don't waste valuable training sessions, especially the early ones where you're developing your basic jet handling skills.
On the other hand, the good TRTOs really will do everything to help you through. They'll pick up problems as they arise and try to sort them. It is very doable, and if you work at it, you'll do it.
btw, I'm not sure whether you are doing this because you want to work in GA, or because you see it as a route to an airline. Either way, I hope you find the job, when you get it (and right now you should) challenging and rewarding and sooo much more fun than scheduled route flying.
Good Luck
the Wings