Frustrating is what it is, we expect consistency and that just isn't possible when healing. Expect ups and expect downs as they're 100% normal, but do look for a trendline over a longer time frame. Don't let bad days destroy you, as they are also 100% normal (but infuriating nonetheless).
A good read on IBD and diets is this, which reviews current scientific research about
diets and IBD.
https://www.dietvsdisease.org/ulcerative-colitis-diet/