Thoroughly disheartened with my score!! (620- 2 month prep) Need Help

So i appeared for the GMAT today and guess what, unlike most of the people on this forum and everyone else said, i had more than 50 points difference on my GMAT prep mock and my actual score. to give a brief background: i started preparing for GMAT in August 2015 but didn't really put my heart and soul in till early november. i also took an off from my office to prepare for the exam in the last weeks of my prep.

My journey in terms of weird GMAT prep scores:

GMAT prep 1: 550 (Q36, V29)
Prep 2: 600 (Q40, V32)
Prep 3: 650 (Q43, V35)
Prep 4: 660 (Q47, V36)
Prep 1 (again) just a day before the exam: 670 (Q45, V37)

Actual exam 620 (Q39, V35)

i want to retake the exam. so i don't want any suggestions regarding whether i should or not. i really really want help regarding how to improve my quants score. Sometimes when i used to go through the posts on this forum, i used to feel like i am the only one who is weak in quants. Every time i read a debrief, it felt like everybody has their quants figured out, which made me feel even dumber coz every debrief (at least the ones i read) started with something like 'i was scoring Q49, ever since the day i was born, so i just focused on my verbal skills'. if something is easy for everyone else in the world how am i only one struggling with it? maybe i am doing something fundamentally wrong. i always knew that i don't have the aptitude it took for maths but i wanted to overcome that with my hard work and i tried my level best. Almost EVERY debrief i read nobody ever complained about not getting enough in quants. on the other hand, my verbal is slightly on the better side. Though by the end of it i could never figure out how to break the V40 barrier.

i want to give myself another 2 months before appearing for the exam again and i really want to do better in quants. i struggled with everything during the exam, managing my time, trying to figure out a way to solve the problem, struggling to come up with a way to tackle the problem and i was hardly sure about any of the answers i marked, especially for DS questions.

Before my rant gets too long to read, if anybody can tell me any strategy for quants, how to begin right from the scratch and how to manage my time and nerves during the exam day, that would really help me. Also, any suggestions regarding breaking the V35-40 barrier would be great as well. Although for now, i think i am just going to focus of improving my quants skills for a month.

People of this amazing forum kindly help my devastated heart. Oh, if it helps in analyzing my failure. Here's a list of materials that i referred to for preparing and thought i was improving but apparently i didn't:

OG - 17 (did it twice, both maths and verbal)
GMAT question pack
GMAT verbal and quants review (although wasn't able to finish the verbal review)
took 4 GMAT prep mocks

Please suggest something which can help build my concepts in quants.

Thanks