Hi Trix and welcome. We all go through the experience you are going through. So you aren't alone on that one. Part of what you said is true, it is a diagnosis of exclusion. Since there are truly no tests, blood or otherwise, that have been successful in diagnosing FM, for now it has to be that way.
As for getting answers to your pain, the only answers you will get is information. There really is no definite "answer" as to why you have FM. It happens for different reasons for everyone. This illness is truly an illness of self-discovery. What meds work for one, may not work for another. What symptoms one person has may be severe and someone may not have it or it isn't an issue at all. In that way it is a real pain in the butt!
My recommendation to you, is like you said, find all the information you can about FM in general. I highly recommend you start with this book Fibromyalgia and Chronic Myofascial Pain: A Survival Manual by Devin Starlanyl. It is what I call the FM Bible and is great in describing bit by bit things that you need to know. Also the best thing to do is what you have already done, reach out to those who have FM and ask questions. Maybe others can identify with what you are feeling and those who don't, offer support.
You aren't alone in this and your frustration is totally understandable. We have all been there.
If you have any questions at all, please feel free to ask.
Again, welcome!