I was diagnosed with IBS but none of the traditional treatments helped at all. My doctor suggested cutting out dairy & wheat, but I kept getting sicker. After months of testing and frustration, I finally figured out on my own that I have a sensitivity to soy! When I cut out dairy I had switched to soy milk, so that's why I was getting worse.
So what I would recommend is starting out with really mild, bland foods, similar to what you would eat while recovering from a stomach bug. Dry toast, broth, applesauce, cereal. Then gradually add in one new thing a day, so you'll know without a doubt which foods bother you and which ones don't.
For constipation, there are 2 tricks that have worked for me better than any specific diet. First, drink TONS of water. Sip water throughout the day, and drink a big glass of water with every meal. Keeping your body hydrated helps with digestion so much. Second, when you eat fruit, eat it before the rest of the meal. If I eat something heavy like hamburger, then eat fruit afterwards, I get horrible stomach cramps for hours! This is because fruit digests much faster than other foods, especially meat. Meat can constipate you, but fruit is high in fiber, so they're fighting each other and backing up your system! So a lot of water, and a little bit of fresh fruit before each meal, can really help.
I also get very bad heartburn, and peppermint tea has done wonders to help with stomach acid & discomfort. And avoid fiber bars like the plague. Good luck!