From a 540 to a 470!

I have been studying on and off for the GMAT for a little more than 2 years now but these past 2 months, I have made a commitment to consistently study for the exam. I have a BA in English Lit so I don't have a quant heavy background like most people in this forum. My goal is to break 600. I would be quite happy if I scored a 620-650 on the GMAT but breaking 600 has eluded me for quite some time and I am getting frustrated and I am feel very vulnerable. I've done the Manhattan Prep course twice and I've written the GMAT last year (440). The highest I've ever scored on a practice test was a 560 (Q36 V31).

I'm not too sure what is going on but my practice scores have been going down recently:
1) 530 (Q36 V28)
2) 540 (Q37 V28)
3) 470 (Q37 V18)
4) 440 (Q27 V24) (I was able to bump up my verbal after I decided to review my S/C (modifiers) again)

I usually study between 18 to 20 hours a week but I feel like the way I approach my studying is not working since my scores have remained stagnant. My biggest problem is that I forget fundamental concepts and techniques quite easily (especially FDP and Algebra). Verbal is not so hard for me and to be honest, I've been getting by without having to go through any OG questions, (but clearly that has to change now). My old way of studying would be to spend 2-3 weeks on one subject but that means I forget the other stuff (NP or WP) that I previously spent 2 weeks on! I don't think this is the way to study. I rarely reviewed my work but I'm just starting to to do that.

What I'm looking for is some advice on how your study schedule looks like. Do you leave two days for review? Do you rotate topics every 2 to 3 days so that you don't forget old material?

I normally don't like asking for help but I feel like I could learn a lot from this forum. I don't want to give up on my MBA dream. I would appreciate any insight or advice. Thank you.