I've had similar experiences. It's kind of terrifying, actually, you have to walk the line of being annoying enough to get what you want but not so annoying they completely ignore you.

One of the things I've learned is to apply the Socratic method, preferably in front of their colleagues if they try to brush you off. Also, if you've got scary symptoms, demand they write down what your symptoms are on your record and why they aren't concerned, then demand a copy of all relevant records (it's your property) -- this often gets them to reevaluate your risk in terms of their career.

Worst case scenario, you can make up medical history. I had a family member pass recently from colorectal cancer. They had symptoms but their PCP kept insisting they were too young. My family member suddenly "remembered" her aunt died of CRC when she was a kid. This got her referred to a gastroenterologist, and they found out she was stage 4...after her PCP delayed her diagnosis for like a year. She actually had grounds for a malpractice suit but who's got the energy or money for that when they're fighting stage 4 cancer?

Also, know your state's agency responsible for policing health insurance. This family member's insurance company routinely tried to deny life saving procedures, but a call to the right govt agency set them in line each time.