Don't let the doctor tell you it's all psychological. That will exacerbate your symptoms, but not cause them. (Stress is usually doctor code for "I don't know") Are you going to your regular doctor or a gastroenterologist? If you aren't seeing a specialist, you may want to look into it. Also, did your doctor test you for a bunch of other things before you got the IBS diagnosis? If not, go to another doctor. IBS is a diagnosis of exclusion, meaning that other things need to be excluded before you can say it's IBS.
As for IBS, food sensitivities can be really critical for some people. I would recommend keeping a food diary and note what you ate around the time you notice symptoms. Caffeine and alcohol can really aggravate IBS, as well, so you may want to try to limit those.
Some people have also had success with probiotics and prebiotics. Probiotics are "friendly" bacteria that can live in your intestines and help you digest food. Yogurt is a good source, but if you go to your local health or natural food store, you will see tablets and yogurts that contain high doses of these bacteria. Just make sure not to mix too many types of bacteria. Sometimes they don't get along and can cause problems, if you know what I mean....
Prebiotics are foods that feed the friendly bacteria. They are usually carbohydrates that contain soluble fiber, such a soybeans, garbanzo beans, and unrefined grains.
The research is still inconclusive about the effectiveness of probiotics, but they do work for some people. The theory behind it is to use the bacteria to help you digest food and to balance your digestive system over all. You can do some research into the topic and see what you think. There is no real harm in trying it, except for the time and money (which shouldn't be very much).
I hope you find something that works for you.