Hi there,
Don't give up. I had a very similar experience. I thought I was doing well on my CAT exams and thought that the real GMAT wouldn't be much different. But on test day I scored 590; well below my norm. See here -
https://gmatclub.com/forum/test-is-on-saturday-talk-me-off-the-ledge-190215.htmlFrom there I did a lot of quant review (as my quant section really hurt me on my test day). I did every quant problem 2-3+ times in the OG until I could solve every problem in under 1:30. I made note of any specific problems or principles that I struggled with so that I knew where to work.
I took some more practice CATs and saw my score improve more, so I was pretty pleased. On my retake I scored 660, which was a little lower than I was hoping, but I was very pleased because I honestly thought that I had done worse on my 2nd attempt than my 1st while I was taking the exam.
For me, I ran into timing problems which caused me to start freaking out. I feel that I try to be very careful on the real exams, and this just drains my clock. So my main advice would be to try your best to relax and stay calm. If I was able to do this, I probably would have scored higher. But I'm content and happy with a 70 point increase. So keep your hopes up and just work towards a retake.
Best of luck,
MDL