Generally speaking, certain foods may make you feel worse, but shouldn't actually worsen the illness itself. There are caveats to that, such as, if you have celiac disease and continue to eat gluten, that will cause damage to your intestines. Or if you have narrowing/strictures, eating foods that don't digest well (like nuts or fruits/veggies with seeds/skin) could potentially cause a blockage. Generally, though, although some foods may cause you more symptoms, they're likely not causing any actual damage.
Have you ever done a food and symptom diary? You might want to try that if you haven't. Sometimes it's not always obvious which foods/ingredients are causing you symptoms, but if you track things in a journal then you might notice patterns that emerge over time. That's how I discovered that garlic is a trigger food for me.
In terms of giving you simple dietary advice - here are a few things that seem to work for most of us. Cook your veggies well - raw is difficult to digest, but boiled or steamed tends to be easier. Remove seeds and skin from any fruit or veggies. Fiber can be difficult, but soluble fiber tends to be easier to digest than insoluble. If you're flaring, think of the BRAT diet - that stands for bananas, rice, applesauce, toast. In general, any bland white foods like that tend to be easy on us - plain pasta, baked chicken or fish, potatoes without the skins, eggs, etc. Many of us can't do gluten or lactose but some of us can - it might be worthwhile to try going gluten-free and/or dairy-free for a short time to see if that helps. Probiotics are worth trying as well - yogurt or kefir is a good place to start. And in general, you'll likely feel better if you make a habit of avoiding processed or fast foods. I'm currently working on that one myself.
