For example, I currently have Fruticose Fried Angler active: I want to consume Ashenbone Bread: But, it won't let me. If I right-click and Consume Now, nothing happens. I also can't cancel the active one. Is it because this is level 90 and the other is level 100?
You can't cancel food/drink and you can't 'consume now' to overwrite a 'better' food/drink with a 'lesser' food/drink. Where better is determined, first by level and 2nd, by stat boost (if they're the same level).
I had to wait the two hours for the food to expire before I could start consuming the new stuff. I could equip the new food just fine, just not consume it. All good now.