You just have to be forthright. Tell them why you believe their diagnosis is inaccurate and have them explain to you why they believe they have the right diagnosis. And if their explanation doesn't cover the majority of your symptoms or what you're experiencing then ask them to explain why their diagnosis doesn't cover said things but is still the right one. They'll get uptight still but it's the only way to get them to understand that their diagnosis may not be correct even though you aren't the doctor.