GMAT Strategy

Hi everyone,
I took a few CATs so far, and my scores span is pretty freaky...
I started in January and here are my scores
First CATs KAPLAN: 390...(I realized I was rusty to the bone...)
2nd KAPLAN: 590
3rd KAPLAN (in real conditions, Pearson center): 620
4th Manhattan GMAT: 470!!!! I still don't get it. Well I bombed the quant section. I found the questions really hard and the calculations cumbersome, resulting in a huge frustration. I did 15 questions, and didn't have a single minute for the others.
5th 800 SCORE: 660.
Then 6th GMAT Prep: 640.
Finally 7th MANHATTAN GMAT: 590...
To be honest, I find the quant section on Manhattan GMAT super hard, mainly cause it's too long and it stresses time management a lot.
I am taking the real one next thursday; I have to say I feel anxious considering the scores...I am aiming for 620-660 range. It's enough for what I want to do and anyways no matter what I do, english is my third language and verbal will take a million year to improve...
Any tips or advise regarding this damn manhattan GMAT sh** that crushed my confidence to level 0?
Many thx in advance.
C

Honestly the best thing is not not worry about any one test. It seems like you have a good number of exams in your range. Study your errors and take the other GMAT Prep exam. If that is in line with your target score, you will be in good shape.