Shortness if breathe,insomnia,neck & lower back pain, low blood pressure. doctors say they can't find what causing my decreased blood flow to my brain?

Low blood pressure?: If you mean that your blood pressure is low, there are many reasons this can happen. If you having body aches and shortness of breath, if you are having numbness and tingling, you may have an autonomic neuropathy. Maybe you should see a Neurologist.

Your brain circulati: How was it established that you have low blood flow to the brain. If your blood pressure is low you will have decreased blood flow everywhere. I agree you need a full medical evaluation and if OK then you need an exercise program to improve your blood pressure, your strength, your back and neck pain and to improve your sleep. The simple answer is that you may be deconditioned.