I have IBS too and it really sucks!
I don't have it anywhere near as bad as you either, it seems!
I've narrowed some of my triggers down to: Full fat Milk, Icecream, Cream, soft cheese any fried/greasy foods also TOTM has an effect. If I get anxious, excited or too hot and bothered (I'm getting paronoid about my wedding day for this too!) they are all triggers.
I used to dread going out for the evening with friends as it usually consisted of Dinner, Wine and dancing with a late night/early morning. Most of the time though I was ill before I even left the restaurant! IBS is a really depressing thing to have and I really sympathise with you.
I've been dieting since January and my IBS is better than it has ever been!
I also had a gall bladder issue last year and ended up getting it taken out, Ever since then my IBS seems better too - Not sure if this was a co-incidence?! it all effects the same parts of the body - This isn't a recommendation! just an observation
Have you been to the doctors and got him to check out your thyroid, blood sugars etc to make sure its not a medical problem preventing you from loosing weight? Maybe your doctor could refer you to a dietian?
I can't advise you about the shakes etc as I have only ever done it once (about 10 years ago) I do Weight Watchers and find it really works for me. You just need to find a food regieme that works for you. Concentrate on just the food part first get comfortable with that and then gradually bring in the excercise part things won't be so hard to stick to then.
Take care mate & goodluck!