Create a spreadsheet of possible illnesses that cause the symptoms you have, and start working through what testing needs to be done to exclude them. A number you may be able to exclude off the bat because they don't fit well or you have recent testing from a physical that would exclude them (e.g. common cancers). Then working with your doctor to get tests to exclude the rest. Have you done a sed-rate test? (Often it is low <3 for ME/CFS, but normally nobody looks at it in that direction, they only look for it being a marker of inflammation if its high.) Have you had a sleep study to rule out sleep apnea for example? Have you checked yourself for orthostatic intolerance or POTS? (Easy to do with a pulse oximeter at home before worrying about official tests.) And so on. You'll find plenty of information at me-pedia to help you along the path of a differential diagnosis.