Sorry to say this but I think you need to go back to the GP. If you tell the Dr you're so bad you can't leave the house, I'm sure she'll listen.
Most GPs rely on the fact that if you don't go back you're probably better - which in your case is certainly not true, so go back and just explain exactly what's going on.
You can't possibly diagnose this yourself, and really, if you're this ill, you shouldn't really be trying to. The FODMAP diet is incredibly complicated and you need the help of an expert dietician. I understand that you're desperate and just trying to help yourself as nobody else seems to be doing so, but honestly I don't think this is the time for a DIY diagnosis - I think you need to see a gastro.
Many food intolerances are complicated to diagnose - you need professional help. No good just cutting out dairy - you need to follow a special elimination diet for a certain period and then reintroduce certain foods one at a time, very slowly to see which are affecting you. It's really hard to do this on your own, and in any case, it might not even be a food intolerance, so please do seek expert help.