I'm not sure how useful my advice will be, but I feel you.
I have been through about 7 years of this, with pretty steady popping up of miscellaneous mysterious, autoimmune-disease-like symptoms, and multiple diagnoses that STILL don't explain all the symptoms. So I've been through a bazillion tests and the attendant stress, often with no clear diagnosis or explanation or resolution at the end of each round of testing.
Like you, I have chronic pain (joint, muscle, soft tissue), migraines, and digestive problems since I hit puberty (I was totally healthy as a child). I also suffered from a suite of escalating other symptoms in my teens and 20s, and was eventually confirmed to have an endocrine/hormonal disorder (PCOS). Once I learned to manage that, my health was then very good and stable until around age 40, when I began developing a whole new suite of problems: vision disturbance, new kinds of digestive problems, worsening pain, chronic utis and urinary pain, unexplained weight loss, neurological problems, intermittent muscle weakness, tachycardia, chronic cough and hoarseness, severe patchy hair loss, skin rashes of multiple kinds. Basically, my entire body freaked the hell out and I was in and out of testing for the past 7 years. In the end I was diagnosed with about half a dozen additional conditions/diseases, which explained a few of these symptoms, but frighteningly did not explain the most worrisome stuff (neurological/vision/muscle dysfunction, etc).
So in the end, doctors still don't know what causes many of these symptoms, and have very little idea how to treat them.
Initially, I was like you...having constant anxiety and always fearing the worst (naturally, b/c as each symptom popped up I was being tested for life-altering things like lupus/MS/Parkinson's etc). And although the worst things haven't been confirmed, in some ways I learned that that is worse b/c I still have no answers to a lot of my symptoms.
However, over the years I went through various stages of learning to cope.
1) Alternating fear/rage. This comes and goes with 'flares' of symptoms. My anxiety was through the roof for about 5 years, and I although I've always had a good 'relationship' with my body previously, I began to loathe it for 'betraying me'. Eventually, as the years went by I began to realize I simply must figure out a way to change my mindset or totally destroy my emotional quality of life.
Cognitive behavioral therapy helped a lot with this, esp the anxiety and the 'anticipating the worst'. Recently, adopting some mindfulness practices have also helped. I wish I had learned this stuff in the first couple of years I was dealing with it, but better late than never. The good news is that I am now far more emotionally resilient to all things in life, not just health problems, than I was when I was younger.
2) Perspective. I have several times had to grapple with the very real possibility of having a serious, progressive, debilitating disease. Thankfully, this hasn't happened, but what truly helped me was to try to move mental state beyond the panic of the IDEA, and to recognize that millions of people have gone through this and much worse, and one day I am almost certain to have to face something like this myself or in a loved one. Therefore, it stands to reason that I should begin to anticipate how I might go on living in a pro-active and positive way, were/when that happens.
Cognitive behavioral therapy helped me here, and I'm certain people on this forum who actually HAVE been diagnosed with serious diseases will have much more useful suggestions than I.
3) Practical but optimistic planning. Initially, my impulse was really to pull back from a lot of things I'd previously done in my life (esp travel) b/c my body was so unpredictable. I have struggled with so many trips in my 40s where I developed puking-bad migraines, ended up in urgent care in strange cities trying to get antibiotics for utis, couldn't sleep b/c of chronic pain, couldn't do outdoor field work b/c of balance problems or muscle weakness, etc. I still struggle with this (my primary response when someone suggests a trip is "are there medical facilities handy?"), but realistically, most of the time my symptoms are manageable enough that I can still do MOST things MOST (if not all) of the time as long as I do some proactive planning (e.g., carry antibiotics on trips, etc). The worst-case scenario I'm imagining usually doesn't happen, and the slightly bad-case scenarios are usually manageable, so I have learned not to worry so much.
4) Practice. To some extent, the longer I live like this, the better I've gotten at it and the less I worry about it. My husband often reminds me that worrying about something that hasn't happened is like paying interest on a debt you haven't incurred. BE FRUGAL! Don't pay pointless interest LOL. This is easier said than done, I know, but if I could advise myself from 5 years ago, I would say: Focus on what you have real control over, and try to let go of the rest. When symptoms flare, try to manage them as best you can, but don't always anticipate being ill...because there are always stretches of time where you feel pretty good coming along eventually. Eat well, exercise regularly, make sure you stay busy doing things that are as fulfilling as possible, remind yourself of all the good things that are still in your life, stop focusing on things you are 'losing' (b/c all of life means eventual loss). View this as 'resiliency practice'.
Most likely, you will have better luck getting answers than I did. So I would advise looking into therapy for generalized anxiety, which will certainly help you manage health-related anxiety. In the meantime, I'd suggest reading the book "Feeling Good" by David Burns (intro to some CBT techniques) and particularly focus on the disordered thought patterns that contribute to anxiety. Also, remember that tons of people go through challenging health issues and continue to lead full lives, even if the lives aren't exactly like they were before. If you are diagnosed, you can seek out people who have very specific experience with that particular condition who can help alleviate your fears.
I really hope this helps. I know exactly what you are going through.
