LONDON (Reuters) - Irish singer-songwriter Sinead O'Connor has cancelled her tour schedule due to bipolar disorder, she said in a statement quoted by British media and fansites on Tuesday. She said she called off plans to tour North America "with enormous regret" and was "very unwell due to bipolar disorder." O'Connor added that she had a "very serious breakdown" between December and March, but ignored her doctor's advice not to go on tour because she did not want to let people down. "So very stupidly I ignored his advice to my great detriment, attempting to be stronger than I actually am. ...