I agree with DJW that a food journal is a good place to start. An elimination diet isn't a bad idea either. For me, whenever I try introducing a new food, I try to do it on Friday evenings - that way, I at least have the weekend to recover if it goes horribly. During the week I try to stick to foods that I know are safe for me. For most people, bland white foods tend to be pretty safe - stuff like white rice, potatoes without the skin, bread and pasta (obviously not those two if gluten/wheat is an issue), eggs, baked fish or chicken all tend to be generally well tolerated by most. Most of us have issues digesting fibrous things like fruits & veggies (some of us do okay with the seeds & skins removed, or the veggies well cooked), and a lot of us have issues with stuff like seeds & nuts, whole grain type things, and some of us have trouble with red meat as well. It's very individual though, some have no issues with any foods and others have tons of food restrictions and triggers. So that's why I wanted to concur that a food journal is a good idea - you can track trends over time and see if particular things seem to cause symptoms. Once you fine-tune a treatment plan that works well for you and start feeling better, you may also find that you can once again eat things that previously caused you trouble.
Since you've mentioned having issues with dairy, have you tried any dairy alternatives? There are things like soy milk, rice milk, hemp milk, almond milk (my favorite - I can't do dairy either). I also really like goat cheese, and it doesn't give me any trouble the way cow dairy products do. There are also things like coconut milk yogurt & ice cream, they tend to be pretty pricey and not everyone can digest coconut (I cannot, it does horrible things to me) but it could be worth a try as well. Have a look around the grocery store or health food store and see what options might work best for you. Good luck!