I have had chronic diarrhea for a few years now and have been told I have IBS due to negative blood tests. But the pain is terrible. What do I do?

Be evaluated. . Ibs is a clinical diagnosis made on symptoms. However many different things may result in chronic diarrhea and if the symptoms are worsening, not controlled and changing than you should be re-evaluated.
Second opinion . I recommend you see another gastroenterologist for a second opinion. Even if the diagnosis is correct, it doesn't sound like you are receiving adequate treatment for your symptoms.
Pain Assessment. A thorough gastrointestinal evaluation would be appropriate by an appropriately trained provider. Negative blood tests by themselves may not indicate ibs. There are many specific tests which can be run and many different medications which could be tried for the treatment of the pain as well as to control the diarrhea.