I took all the written, and will have to retake both Instrument and Commercial written. I'm close to finishing, but will not be finished before my written exam scores are expired.
Personaly, I learned more from practacing. I too felt like I just memorized the answers. Don't worry, you'll understand the material once you start. Three books that should be used to help fill in the gaps: Instrument Flying Handbook, Instrument Procedures, and the big Jepp. Instrument/Commercial.