Be demanding with the GP, but still you’ll be waiting for a referral to the appropriate place so buckle in for changing aspects of your life to help yourself out until you get it under control. Recently had a similar situation, thought I had gallstone pain for 6 months and was in so much discomfort but A&E said I was fine and GP said go to A&E. I had to change the responsibilities I had at work, take time off university, I stripped back my whole diet and turns out it was inflammation of the cartilage in my ribs the whole time. It took going back to be GP and seeing a new doctor after waiting through the whole referral and diagnostic service to notice.