Sinead O'Connor at an "Albert Nobbs" screening in December. (Charles Eshelman/Getty Images)

After canceling her upcoming tour Monday, Irish singer Sinead O'Connor took to Facebook to explain why. In a wall post, she apologized and said she was "very unwell" because of her bipolar disorder.

The announcement came on the heels of a public breakdown for the 45-year-old singer famous for her 1990 hit "Nothing Compares 2 U." Last December, O'Connor married for the fourth time and then split with her new husband 16 days later. In January, the singer-songwriter sent out desperate tweets to fans as she tried to locate a therapist after she'd tried to commit suicide,  according to the U.K.'s Sun.

 O'Connor revealed five years ago on "Oprah" that she had bipolar disorder and had previously attempted suicide in 1999.

In Monday's post, the singer said she had not fully recovered from her winter breakdown.

"I had been advised by my doctor not to go on tour but didn't want to 'fail' or let anyone down," O'Connor said in her post. "Very stupidly, I ignored his advice to my great detriment."

O'Connor's new album, "How About I Be Me (And You Be You)?"  was released in February.