Yes, the unpredictability gets me too. My IBS can be fine at times, and my gut can feel 99% normal! That can go on for days, or even sometimes a few weeks. Then all of a sudden it comes back again. All for no reason I can understand.
I never make the mistake of changing my diet or eating something which could be dodgy for me. I've learned what suits and what doesn't in the past 4+ years, and I stay away from anything I have found repeatedly to be upsetting for my tummy.
So that makes the cause of IBS coming and going baffling. I am not convinced mine is stress related either. It can be sometimes, but sometimes IBS will return when I have nothing stressful happening. I thought it was weather at one point, then that theory didn't really work out either. Mine isn't hormones, as I'm over 70, & 20 years past menopause. Neither is it apparently sleep quality related.
But how do I manage its unpredictability? I am fortunate that even at its worst I don't think my IBS is as bad as some people get. So I don't get the dreaded loo-dash (well, I got that only once), or incontinence, or really very much pain. And I am able to eat a pretty healthy diet with some restrictions, but it's healthy.
I guess I just accept and soldier on and endure when a bad time hits. I take a herbal tincture which can help a bit (Tormentil) and radically adjust my diet to really very binding-type foods for a few days till things get steadier. But fortunately most of my life is quiet, I am alone in beautiful countryside, am retired, and I am not expected to go out to restaurants, on road trips, etc. I have lovely friends, but no "social pressure".
So I can nurse myself quietly at home if I get a flare or a bad time.
When it goes away, I have no idea why, but keep the same steady diet that usually suits me so don't start eating chillies and onions etc!! I never fall into the trap of thinking I 'm better now, and it's gone so can eat what I like. I learned my lesson that it always will come back (for me anyway.)
That's more or less how I manage. But sometimes I have felt very down indeed with IBS.