Had similar issues with the Flex. My advise, use the factory backing plate, after market plates get bad reviews and tend to fail easily. The amount of power a Flex has with the forced rotation makes it harder to control, go slow and be sure to hold on but not too tight. If you white knuckle it your hands will hurt and your arms will get sore faster. I was able to get good results and finish full details without getting worn out as much with the Flex over time. However I ended up buying a Rupes DA and it is my go to polisher for everything. I only break out the Flex when a panel or vehicle is completely trashed and needs the extra strength or when fixing deeper scratches/swirls.
There are no tricks or anything like that, just takes time to get used to. I also recommend NOT using full flat faced pad, they seemed to be more prone to wobbling.