Unfortunately, FM is a diagnosis of elimination. If you have had negative results for everything from underactive thyroid, to lupus and arthritis then you may just have to accept this dx. There's also the 18points of reference on the body, you need to have pain in at least 11. The pain is usually categorised by being in all quadrants of the body.
A rhuematologist is usually the most qualified to dx and treat it. It is also a very individual condition. I get away with minimal pain killers while others are out of work and taking morphine. It took me about 6 years to accept i had it and i wasted a lot of time and money chasing my tail because of it.
You can really only decide for yourself if you think you need to get more opinions. But remember, the medical community is divided somewhat about it. Some say it's a soft/connective tissue problem, some believe it's to do with faulty pain neurotransmitters and then of course there's the ones who firmly believe it's all in our heads.
Getting embroiled with different medical opinions will only worsen your anxiety, increase your self doubt and ultimately end up not making one bit of difference, except wrexking your head.
I think it's a lot to do with what we put in our bodies and the human body is unable to cope with the amount of junk we absorb from food to environmental pollutants. That's to say it makes it worse, not necessarily the cause.
Anyway, you're not a nut, your just going through the same as the rest of us did at some point. Make up your mind if you can accept it, when i did it made it so much easier to get on with things. It didn't make to pain go away but at least i could eliminate the mental torture i was putting myself through.
The anxiety you're suffering may be from not feeling in control of your body, not getting answers and feeling you can't trust the medical profession. Again, only my opinion from experience.
I hope you can get some peace, take care,
Fibromyalgia since 2007