So this is actually a really common place to get stuck, and I remember being in almost exactly this spot before things clicked for me. Feeling busy with prep but not improving is its own specific problem — it's not about working harder, it's about working on the right things.
The single biggest unlock for me was building an
error log. Not just "I got this wrong," but a proper categorization: was it a concept gap, a reading error, a wrong approach, or just carelessness? Once I started doing that for every wrong question, patterns showed up within a week. I had no idea how often I was misreading Data Sufficiency stems until I could literally see it in a list.
For structure, I'd think about it in three phases: diagnosis, concept repair, and timed practice. Right now it sounds like you're doing all three at once without a clear sense of where you are on each. A useful starting point is the GMAT Club diagnostic quiz — it'll tell you which subsections are actually weak vs. just unfamiliar.
On approach: if you understand a concept but still get questions wrong, that's usually a strategy problem, not a knowledge problem. For Problem Solving, it often means you're not identifying what the question is actually testing. Take your last 20 wrong PS questions and ask yourself: could I have figured out the approach in the first 30 seconds? If not, that's the work.
Mentors/tutors can help a lot if the issue is specifically that you can't diagnose yourself. But try the
error log approach for 2 weeks first. You might find you don't need the external guidance once you can see your own patterns clearly.