I seriously need some advice

So I've been preparing for the GMAT more or less constantly over the past nine months. During that time I worked through all of the MGMAT guides and did every single question in the OG including reading all the answer explanations. I took an initial diagnostic test before actually starting to study and scored a 580 (Q43,V28). So far so good, I was thinking that an above-average score without preparation is not that bad and having in mind that an improvement between 100 and 150 would be doable with a decent preparation according to bb, this is acceptable.

On the way I had serious motivational issues just because of the huge amount of material and I wanted to keep the other simulated tests till the end when I've covered every area of the GMAT. Often times it was more working in a delirium because my progress was so slow. Although I don't have an overview of how much time I spend preparing, I would estimate it to be somewhere between 300 and 400 hours. Ridiculous. A total overkill.

Just by the end of last week I was done with the material - finally - and excited to see if my studying was of any use. So I started with one of the MGMATs and scored a 670 (Q45,V36) and in the first place I was really disappointed because if you read a couple of the debriefs and experiences in this forum, you'd expect the average to be at least 700 and that everyone is actually able to score 700+ with an appropriate preparation, no matter what your mental capabilities are. I was hoping that the following test might be better, just because I would've been used to the test setting by then. I scored a 690 (Q46,V38), which was quite close to my actual goal and I was already happy, although I would be at the far lower end in the debrief section of this forum. After reviewing the questions, I was ready to take the next one and got back to the 670 (Q45,V36), yet blamed it on being tired when I took it.

My next great idea was to take the GMAT prep tests again and this is where my GMAT story begins to worsen. I was hoping to finally crack the 700, as virtually everyone here scores higher on the GMAT prep tests as on the MGMAT tests. Already during the test I felt that the verbal part was way harder that the MGMAT equivalent and I had lots of trouble with the timing. The score screen showed up and it was a 650 (Q48,V32). My state of disappointment reached a new level and I was puzzled as to how I would score a lot higher on the MGMAT tests. So I decided to take another GMAT prep today to see if it was maybe just an accident and I would return to my 670-690 range not expecting to score higher like it seems to be normal. I just finished the test and now it's not just disappointment, I'm rather devastated. It's been a 620 (Q42,V34). So my quant score dropped actually below the level before I started reading those gazillions of pages. I don't know what it is with me and the GMAT prep, I'll try to do another MGMAT test to see if I would start to climb up the ranks again or maybe it's just that 4 tests within one week were too much and I'm burned out or something. I also noticed that I've trouble reading as fast in the GMAT prep as in the MGMAT tests. I don't know what the reason could be.

I was actually planning on taking the real one next week, but considering where I'm at now that's not realistic anymore. I don't know what else to do. I pretty much did everything: timed every single OG question, have an error log, tried to grasp the concepts as much as possible, but it still doesn't work for me. I really think most of the people in here are real geniuses, even those who pretend not to be.

Well, any advice is welcome because I don't know what to do next and if I would score a 620 or even 650 on the real test, then this whole drudgery would've been in vain and I could've done lots of more entertaining or productive things the last couple of months.

Cheers.