6 months of studying with no results.

Hello Manat
You've received some really valuable advice from many people here already.
I just want to add something that has helped me personally (and not necessarily will help you, but you may meditate over this).

I am in a similar condition as you (confident about improvement in verbal, but gradually improving in quant with practice). My scores are similar to yours.
There's 1 difference. I'm not giving mocks at all right now. I gave one 3 months back with a score of 530 or so. It was with only 3-4 days of studying.
What I realised then is that scores can be improved with targeting 1 sub-section at a time. E.g HCF-LCM in Number properties, then Divisibility and Remainders, etc.
>>So here's what I'm doing:
I achieved accuracy of 85% in 600 level questions of 1 sub-section and then another 3-4 days of practise (sometimes more) and error analysis to achieve 70-80% accuracy in 700 level questions.

ONLY at this stage, do I move to the next sub-section and then I take an assessment of mix 600 and 700 level problems of the main section (e.g Number properties) using GMAT Club questions. Then repeat the process with Algebra.

What I found is that I am really confident in facing tough Number Properties questions now. So when they turn up in my mock test later, I'll be better equipped to handle them. Same should happen with Algebra, etc.

The point is to be absolutely thorough with one topic and only then move to the next one. It's a little time consuming in terms of "Oh it has been 10 days and I'm still doing number properties", but the process has been worth it for me and I feel I'm improving the quant score dramatically.

You may try this method and see how it works for you.

This method was recommended to me by e-gmat, which as they claim, is proven amongst thousands of 99%ile scorers. I'm simply following what successful people do and SO FAR I am extremely satisfied. I'll give the mock only once I'm at 80%+ accuracy in 600 and 700 level Qs, so that there's lesser chance of disappointment - and all I'll have to work on is time management then.

The point is to leave the mocks for later and use practice questions to figure out where you're going wrong and absolutely hammer the right concepts and approach in your head.

Hope this helps.