Hi muggergammy,
Everyone attributes this to a discipline problem but it's usually something else: FEELING A LACK OF MEASUREABLE PROGRESS toward your goal.
Practice tests aren't good measures of steady progress. You need to do them to keep yourself aligned to the test you are going to take.
We use a tool we call your REAL TIME READINESS (RTR) Score. It measures progress not just in your knowledge of core principles (content), but also how your in-test process is improving (how you're getting better at your approach to problems in Quant and Verbal).
Our FREE 90 minute Readiness Assessment Session is available to help you create new energy in your prep. You'll see why your motivation is off even though your MBA / career goals are important to you.