I had IBS for over 20 years, but I finally got it under control two years ago. What worked for me, might not work for you, but I know it's worth a shot. Since I've been there, done that, I know almost anything is worth a try.
My doctor claimed it didn't matter how much I used Immodium. I'm sure that's true as far as it not causing cancer or whatever. But I think it contributes to IBS symptoms when used regularly. It perpetuates the cycle of constipation and diarrhea by creating constipation. Even if you don't think you have constipation, you do if you have IBS. Gas and diarrhea back up behind it and break it down or blows it out. Stopping the use of Immodium was the first step in my resolving my IBS.
The reason different diets didn't work is because your IBS isn't caused by one diet factor. You've figured out that you're not lactose intollerant, not sensitive to gluten, etc. (I did all the diets too.) It's a combination of factors that will ultimately result in a individual diet that is specific to only you. I had to go through many steps before I figured out my individual diet.
When I was the most miserable, I broke down and did a bland liquid diet for a few days---like you have in the hospital after major surgery. Then I moved on to a bland soft diet for a few days. Then I moved on to bland regular food. No processed food. Just fresh fruits, vegetables, meat---without many seasonings and keeping the fat away like butter, etc. I did this for about a month or a little longer. I needed to give my digestive system time to settle down.
I already knew the main foods that triggered it. I'm not lactose intollerant. I can drink milk without a problem, but I can't have ice cream. Ice cream and milkshakes KILL me. It's so painful. Too much cheese is a problem. I can have cheese everyday---I just can't eat cheese all day. (It's something I used to grab out of the fridge to snack.) I can have bread everyday---I just can't have lots of bread throughout the day. (It's another thing that I tended to eat often.) I can do one or two slices of bread TOPS. I slowly added in seasonings and spices. Only garlic gives me a problem, but only on garlic bread (KILLS me like Milkshakes)---I can still cook with garlic. Those are some examples.
Schedule and portions make a difference. Eat too much in one sitting and my stomach hurts like hell. Skipping meals is bad too. My body likes a consistent schedule of meals in reasonable portions. The normal three meals per day with one small snack inbetween.
Getting off caffeine was important. Carbonated beverages were important to eliminate too. Soda errodes the intetestines and contributes greatly to IBS.
Adding FIBER was the solution that really pulled things together for me. I learned that there's what I call soft fiber and hard fiber---fiber that absorbs water and fiber that doesn't. Water absorbing fiber is fruits and vegies. Fiber that doesn't is nuts and grains. I had already added fresh fruits and vegies to my diet, but I needed something that I could eat anytime, anywhere---to balance a beagle or something I grabbed to eat while at work when there weren't alternatives. That solution was having nuts and raisins always on hand. I'd make sure to eat nuts and raisins with each meal or as snacks during the day.
The reason fiber is important for IBS is because it helps move everything through your system. When you have diarrhea, everything is moving too quickly and it needs to be bound up with fiber---to absorb all that liquid. When constipated, everything is moving slowly and it needs to be moved along with fiber. Balancing fiber is important and rather tricky with IBS because you need a good balance of both types of fiber. I found adding raisins and nuts helped keep things balanced if I eat them regularly with other foods.
Once I became super constipated and my stomach swelled up (like everyone, I sometimes screw up y diet and eat too much bread out of laziness or whatever.) I didn't take medication. I fixed it naturally by eating tons of fresh fruit for a week. It takes patience to do it right. Forcing anything will cause you to get back into a terrible swing cycle.
If I feel like I'm just a tiny bit constipated, I'll drink a very small amount of caffeine (half a glass of pepsi----just once) because caffeine is a natural diuretic if I can't increase my water absorbing fibers because they're not handy. (This pepsi solution is only for RARE occassions, you can't drink pepsie everyday.) I also still take Immodium on very RARE occassions. Again, this happens when I get out of my groove and mess up my balance---eating to much of what I shouldn't, eating too much in one meal, or skipping too many meals.) I won't take it right away---I'll try to fix it myself by adjusting my diet, but I'll take one dose of Immodium if it continues for a few days or after only one day if it's a particularly terrible day. BUT I'll also increase my fibers at the same time so the Immodium doesn't cause constipation.
STRESS. Anxiety is a huge contributor to IBS. Try to become a more calm person. Try not to worry about things. Try yoga. Eliminate things that stress you out. Go to therapy if necessary.
EXERCISE: Exercise is important for two main reasons: It helps relieve stress and it keeps your intestins working. Exercise helps move food through your body. Don't believe me? If you're ever constipated to the point it hurts, try stretching your torso---arms in air, moving side to side, touching your toes, stretching backwards, doing situps (I know it hurts with all that gas), doing the downward dog (
http://www.alaskaartguild.com/Yoga/images/5.jpg) and you'll be blowing gas and things will start moving. By exercising regularly, you won't need to do these things to solve a problem. Regular exercise will help keeping things moving.
My IBS symptoms have been almost non-existent for two years doing this. Don't get me wrong. I still have IBS. I'm managing the symptoms. I pay attention to how my body feels and I adjust accordingly before it gets out of control. As a result, I don't get bouts of diarrhea or constipation unless I make a mistake and I'm able to correct the mistake quickly. Granted, I can't make mistakes every week. That's not stable enough. But a few times a year, I make a mistake but the symptoms are mild because I figured out how to adjust everything to fix the mistake.