Unfortunately, when airlines don't hire you, they usually don't tell you exactly why. With your time, I wouldn't waste money on an RJ course...airline training is just that, and THEY PAY YOU for that. What you might want to do is apply for and interview at airlines you have no desire to actually work for, and use that as practice. Then reapply to the airlines you really want to work for, or others that are still good companies. That way you will know what you need to do to pass the interview, roundabout.

The big thing is to be yourself and don't put on an act, and attempt to relax!

Good luck!