Sorry to hear about the low score.
The same happened with me about a fortnite ago.
However, mine was a bigger disaster than your's.
I was scoring consistently above 700 in mocks but ended up with mere 480 on actaul GMAT.
So here is the deal.
The GMAT tests many things not just your knowledge of material.
Your attitude, test taking skills, approach and timings play a vital role.
In my case, negative attitude and nervousness were the major reasons.
I would suggest revise your basics and spend quality rather than quantity time on study.
Take ample practise mocks and spend double time analyzing the results.
Staying calm and confident are key factors.
In the end, it's just an exam.