Howdy. My last math course was 7-8 years ago (in high school), and I don't recall fairing too well in it. I've always hated math, and done poorly in it. My HS math GPA was probably a 2.5. I have a feeling I am not as poor at it as you, but we have similar stories here.
Anyways, since I knew QR would be my worst section, I attacked it. I used the same tools as everyone else, Chad's Videos and Math Destroyer. I also used Khan Academy for trigonometry. I felt Chad's videos were a waste of time in QR, but most people enjoy them.
The key in QR is diagnosing the type of problem and recalling if you know how to do it. Therefore, in our preparation we need to understand the fundamentals of each type of problem we do. You have to know the basics so they can be applied in unique ways.
I did the 2012 Math Destroyer and timed all 13 tests. Each person is different, but I would never dream of not taking timed tests, simply to understand the pressure it applies. I averaged 30/40 on Math Destroyer in the beginning, but that lowered as the tests got harder. There were two tests where I only answer 13/14 problems correctly, and this was like 2 weeks before my test. Needless to say, I stepped up my game.
I felt Math Destroyer was the best tool because it forces you to apply multiple concepts in each problem. This means that each question is usually actually 2-3 questions. This tests your true conceptual understanding. If you struggle in this book, then go back to basics.
My "dream score" was actually a 20, and my practice tests were averaging around an 18. I made a 23 on the real exam, which means I missed probably 2-4 problems. Recall, I was getting 13/40 on Math Destroyer 2 weeks before my test and scored in the top 98%. It can be done in 3 weeks, but you have to focus and eliminate sources of weakness. It's no different from any other type of studying.
Now is not the time for fear, it is the time for action.