It sounds like you have been prepping for about 18 hours a week, which is a ton. Sustaining that over ~6 months is ~430 hours of prep, which is completely unnecessary. No one needs to prep that much if he or she preps smart.
You are right: because you are losing motivation and prep quality you should lower your amount of prep. Your proposed plan of about 13 hours a week sounds good, but I recommend taking at least one off day each week. That will really help boost your motivation and help you have more productive prep sessions.
When you are preparing as much as you are, it is really important to make sure you are prepping intelligently. That way, you are not just putting a ton of prep hours in, you are improving a great deal from each hour. My book,
GMAT Clarity, will help you do exactly that.