I am so sorry you are having such a rough time. And I'd go with what the gastroenterologist says over the GP anytime your gut is involved. The only thing I see you haven't mentioned that you tried is the low FODMAP diet. There are lots of resources on-line but the best is the Monash University in Australia appears to be the experts. They have an app that has a one-time fee of $8
and lots of folks with IBS get good results with this. I did not, but then I have IBD, not IBS. Worth a shot if you haven't already tried it.