IBS is actually a neurological (interaction between gut and brain is disturbed) and functional/physiological disorder, not a psychological one. Stress can aggravate your IBS symptoms but it is not the cause of IBS.
Anti-anxiety pills sometimes work because they tranquilize the gut and those awful spasms. Certain medications, like buspirone, are thought to reduce the sensitivity of the sensory nerves to what's happening in the intestine.
The fact that your doctor told you not to think about it seems like he doesn't know what IBS is or understand how to treat symptoms of IBS. I'm sorry he's not taking your symptoms more seriously.
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Re: Can IBS symptoms be caused strictly by lack of sleep or stress? *rant warning*
