First Real GMAT lower than first 'No Prep' GMAC Official test score

Hello,

Today I did my first real GMAT test, but I only scored 550 (Q40, V26, IR 8). I was shocked when I saw this score (deleted it); as my first mock test score (GMAC official test 1) was 580 without prep. I have to say that I lost it very early in the test, especially Verbal was a real nightmare for me. My first three questions in verbal were Sentence Correction, which is by far my weakest segment (I struggle even with easy questions in sentence correction). After that I got two RC extremely long passages about nature, which led to me having a timing deficit. Also quant is a lot harder on the real test than in the mock tests, one of my final questions was for instance one about a sphere in a cube (which is categorised as a HARD 65% question on GMAT club); while my score is only 550 (this was not a sub-600 level question?!). I have to conclude that I find the real GMAT significantly more difficult than the official mock tests, in both verbal and quant.

The only positive point for me was the IR score, which I see as OK for someone who didn't practice IR at all. But, this segment doesn't matter at all for the programme I want to apply for. I only need to score 600 as a threshold score for admittance into a particular program.

History:
GMAC Test 1: 580 (Q44, V25, IR 0) 18-11 - almost no prep, only familiarized myself with DS and PS with +- 50 questions in the Q OG guide. NO verbal prep.
GMAC Test 2: 600 (Q42, V31, IR 0) 09-12 - I was disappointed with the progress I made after prepping for three weeks.
GMAC Test 3: 640 (Q46, V31, IR 0) 10-12 - I was happy with this score - significantly above the score I needed.
GMAC Test 4: 580 (Q43, V27, IR 0) 13-12 - Lost a bit of confidence after seeing this score (made a lot less errors than with the 640 score, but I missed the first 4 questions in Quant and the first 3 in Verbal).
GMAT Real Test 1: 550(Q40, V26, IR 8) 14-12

More statistics; I prepped with all the OG 2020 guides, with about 750 of the questions answered this is my error log:

% Correct % Incorrect
DS Easy 90% 10%
DS Medium 57% 43%
DS Hard 69% 31%
DS Very Hard 25% 75%

PS Easy 74% 26%
PS Medium 69% 31%
PS Hard 50% 50%
PS Very Hard 55% 45%

CR Easy 81% 19%
CR Medium 68% 32%
CR Hard 69% 31%
CR Very Hard 56% 44%

RC Easy 89% 11%
RC Medium 71% 29%
RC Hard 50% 50%
RC Very Hard - / -

SC Easy 78% 22%
SC Medium 42% 58%
SC Hard 65% 35%
SC Very Hard 45% 55%

I really don't see how my GMAT score compares to how I relatively scored against the OG quide questions I answered. What I find weird is that, on average, I tend to answer more hard (generally perceived) questions right than medium questions?

The areas where I need to improve are, based on my own findings:
Sentence correction - When reviewing my mock tests, about 80% of the verbal errors I made are in sentence correction. I really don't see why one answer is more correct than the other.
Reading comprehension - Without having the ability to mark certain key passages in the real test (like you can in the book), I struggle with large texts. Does anyone have a trick for this?

I need to have a 600 score within the next 2 months. Can anyone give me advice in how to tackle this issues I 'created' (scoring lower than my first mock test)?