I agree ^^ I would spend 1 solid year at a level before moving up.
Jim Wofford and David O'Connor wrote some great articles on how to decipher when you are ready to move up, I know I read them in the PH Magazine, I have to dig them up - but they both give great tips.
Don't rush yourselves - when you do that, accidents start to happen because you discover you are overfaced - so take your time. Enjoy yourselves and just have fun where you are comfortable.
Important Rule Of Thumb though - your Trainer should be working with you, 1 level above what you are competing *when you are ready* So if you are competing at BN, you should be confidendly schooling over N.
While dressage is important - due to the short formats....it isn't the be all and end all at lower levels. Nelson and I can place 5th after dressage, and still take 1st after CC and Stadium - but the higher you go in levels, your dressage score can and will make or break your placings. But, conquoring personal goals is more important than winning a ribbon.
Focus on all 3 for sure - learn to be solid, learn to be confident, learn to make mistakes and correct them.
Here is a great radio discussion with Boyd Martin that'll help you - start at 13.29 minutes.... Moving Up the Levels and the PRO-AM Status - Eventing Radio Show