All I can say is I feel for you and know this very well. I also get something that seems to work to help me and I feel much better, and sometimes for even weeks! I just start to get used to NOT having IBS any more and truly believing it's "gone away" (thanks to whatever it is I tried)...when it comes back all of a sudden and can linger for weeks. It's so disheartening.
The only thing that gets me through, still sane, is awareness that if it went away once it can do it again, and all I have to do is hang on and wait. Sure enough eventually I will get another "remission"...maybe for a day or two, maybe for a week, or even longer. The sad thing is I know if it came back before, there's always a chance it might come back again. And sure enough, in its own time it does.
It feels like a devil who plays nasty games with me.