Work Plan, Methodology and Hard Questions

Hello Gmatclub community,

After one month of anarchic preparation I took a Manhattan Gmat blank Test and scored a 550 (Q38 V28), which was disappointing but deserved considering my preparation.
So I decided to change my approach and methodology to be much more rigorous regarding my preparation. After some research I found this post : " a way to increase from q35-40 to q 47" (sorry I can't post any link yet).
This post helped me a lot to develop a good methodology, basically what I do is go through the concept of each Manhattan Gmat book and time myself doing questions and then I do all the mid test and final test in easy, normal and hard mode. I also started watching Ron’s video.

I just finished the Book n2 ( Algebra ).
I have no problem with the easy and normal Tests, I finish on time or ahead of time with 100% accuracy but when I try the Hard set it really is a disaster... I have no clue on how to tackle most of the problems, I get overwhelmed by time pressure and only manage to have 50% accuracy with lucky guesses...

What is wrong with me ? I don’t understand why is there such a big difference between Hard Set and Normal Set ? What am I missing in my preparation ? Am I just to dumb to manage to get right the hard one? Is getting a 700+ is delusional ? Or is there some kind of methodology and mind set I need to develop?
How do you manage to get right the hard question for the first time you see it ?

My biggest problems are :
- No clue on how to tackle most of the hard questions
- I found Testing Case to be a huge loss of time, I know this is really important and I need to master it for the DS part but every time I need to test case it takes me a crazy amount of time to prove a point (like 3-5min) so i get confused thinking of all the time I am losing and end up skipping the question or get it wrong ... It is really frustrating

Do you have any advice on how to tackle tricky questions and DS with test case ? And also any advice regarding my prep plan, what can I improve ? (see below my detailed prep plan)
I thought that after finishing the concept book I would be able to have a good overall score but damn it is so frustrating to look at a question and be totally lost ...

Thank you very much for your help

Regards,

Coggi

PS : Detailed prep plan :

Week : 4h/day Monday- Friday : Manhattan Prep Gmat Book to get the concept + 1 Ron Video every day
Week-end : Manhattan Prep Gmat Book, Mid term and Final Test (easy, normal and hard mode) + Error Log

I do 1,5 Book / week approximately

Thanks again, I hope you guys can help me to score a 700+ ( I plan to take the Gmat at the end of September and if I am not ready/my score is not good enough I plan to retake it in mid-December)