I can do math but I cannot do the GMAT, and I do not know why

Hello,

I need a bit of advice. I've been studying for the GMAT for some time and have not been able to overcome a certain hump.

Now last month I took my first GMAT and scored 640 (41Q/38V), and was properly disappointed. The verbal score was fairly low compared to practice tests because I wasn't properly prepared for how tired I'd be taking an exam at 8:00am. But Quant was very disappointing - I have advanced / honors math in high school, studied Finance in college, and have done very Quant intensive consulting.

I. CAN. DO. MATH.

The issue I am having is translating my timed problem sets to the practice tests and translating that to the GMAT. I spent a great deal of time working to understand the underlying "math" and reasoning across many of the subjects in the Quant section of the GMAT. I had little trouble absolutely crushing the problem sets. I tried to replicate exams conditions as best I could. Timed. No notes. Mixing DS and PS across various difficulties. It was not always perfect but after weeks of work (MGMAT quant studying, MGMAT problems set, problem review, GMAT club questions, OG 2016 problem sets, Magoosh and MGMAT video explanations, etc) I was incredibly confident and starting doing practice tests.

Unfortunately, that confidence did not translate to MGMAT practice tests. For some reason, once the "pressure" is on I stop trying to reason my way through the quant section and it becomes a math test. I try to brute force Algebra my way through it. Estimation, backsolving, test cases, etc. all go out the window and for some reason I lose focus. Instead of talking my way through the problems like I do in the library (under my breath, of course), I instead try to do the math in my head, which costs me precious minutes and completely derails me.

I do not know why.

On my practice tests I am perpetually at 40Q but on timed problem sets (across various difficulties) I do well. I do not know why I'm becoming a poor test taker when it "counts".

So since my GMAT I've revisited my GMAT content knowledge (using MGMAT and GMAT Club Book) and, again, I'm doing well on timed problem sets. But I took another MGMAT practice test on Saturday and again, predictably, I got 40 and after review I could not believe the questions I missed. I missed a Geometry problem that focused on the determining the perimeter - I actually laughed because on review I was able to get the answer within 30 secs. But during the practice tests I was sweating absolute bullets.

I do not know where my gap is, and I do not know why I'm regressing during the practice tests and actual GMAT.

I've have done everything top-to-bottom that has been suggested, but at this point I just feel plan dumb.

Any help would be great.

I just don't know what to do at this point because I'm not improving when it counts. I plan to retake the GMAT on the 23rd, and for the first time in my life I am not confident in my ability to improve.