560 to 640 in 4 months, am I just dumb? (not sure what to do)

Just completed my GMAT exam. I thought I should write a little debrief and ask for some opinions. I canceled my score of 640, which I achieved (if you can call it that) after studying for 4 months with no other life obligations. I don't consider myself a "smart" person, my math isn't strong, and my strength in verbal is only due to a passion for reading. You're all geniuses here compared to me, and I'm honestly not sure if I have it in me to achieve a 700+ score after that miserable performance.

A little background. I just turned 25 with an undergraduate honours degree in Economics from York University, with a 1.5yr work experience at BMO as a teller, then Financial Services Manager. I quit in December to study the GMAT full time and to pursue other employment with some of my old professors, which I've been continuously putting off due to my commitment towards the GMAT. They've been sympathetic, I **** up. My first language is Mandarin which I speak and write fluently. My math is extremely poor even by liberal arts standards, and even more embarrassing is the fact I'm Asian. I actually don't know any Asians with worse math than me. I do speak English without an accent, and I love reading and writing. I wish they somehow could test a little bit more for that faculty. I took the GMAT in late December of 2016 after a month of studying and got 560. It put the exam in perspective for me and I now had a good idea of what I needed to do to achieve a high mark, or so I thought.

I couldn't remember my exact score as I was so disgusted looking at that "640" number. I remember getting a 59 percentile in Quant, so I must have done better in verbal. Verbal is actually my forte, as I've always reading. Sentence correction does throw me off quite a bit though, but I can tackle even the most difficult reading comprehension passages and critical reasoning without any issue. I do however suffer from an inability to focus for long periods of time and lack of mental stamina, as I found out the hard way today.

I picked off in early February, starting from what I considered a rock bottom in my Quantitative capacity, with relatively strong verbal (my sentence correct was and still is weak). By late March, I finished all OG textbooks by March and took my second GMATPrep test, achieving 660. I thought if I gave it another two months of consistent effort, I would just scrape by with a 700~710.

I started branching out to third party studying material, reading through the Manhattan textbooks and completing many CATs by GMATClub and Manhanttan. I did terrible on most of them, with the Quant portion throwing me off completely. I could only achieve 620 on my Manhattan CATs, and fairing slightly better on the GMATClub tests. Nevertheless, I plowed on, continuously familiarizing myself with the OG material, and getting thrashed by third party quant material whenever I had the confidence to branch out.

By June, I felt like I've had just about enough. I booked the test, and spending most of my study time on making sure I was comfortable with all OG questions I could get my hands on, and improving my Quant through GMATClub.

I didn't fall asleep last night due to the pressure, stress, and poor sleeping habits. I arrived well caffeinated, in a terrible mood, but determined to tough it out. I felt like I dedicated too much energy to the AWA portion, writing what I felt like was a 6/6 essay. I got 8 on the IR, which felt accurate because I was pressed for time and ended up guessing the latter 4 questions.

I began quant in the worst way possible, without the faintest clue how to solve the first three. I guessed, and the questions got easier. I said screw it, and did what I could. Some calculations took way longer than necessary, I guessed my way through about 6~8 questions, feeling confident on maybe 25/37.

By the time verbal came around, I was exhausted. I couldn't develop my logic as sharply on some of the critical reasoning questions, whereas I'm used to thinking circles around each choice with time to spare. Reading comprehension wasn't bad. The boring topics always take a while, but patience and reflection never fails me. I did find myself spending 2~4 minutes on some of the SC questions, stuck trying to decide between two choices. By the last 15 questions, I was thinking at 50% of my usual capacity at best. The score of 640 was a bit of a shock, I was hoping to get lucky, but deep down I knew I screwed up.

So that was my GMAT experience. I'm glad to take a break from it, but I'm seriously questioning my capacity for this. Any thoughts and opinions by you guys here would be great. Many of you achieve scores beyond 700 and are still unhappy, so I'm sure and any tangents of your thought would be valuable lessons for me.

Special thanks to Brunel. I don't know if I'd actually have a chance at this at all if not for your posts.