What can I do to make it all better?

My partner of nearly 8 years , father to our 3 lovely Boys has suddenly said he is leaving us ! i am absoloutly devasted and cant see how i would ever cope withought him. We have had lots of bad times through out the years. But have always got on with things and been fine. I cant understand why he would do this, hes saying it wont be easy, but he wont change his mind. Ive pleaded with him, its like sonthing really bad has happend, but it hasnt ! i cant imagine being on my own, and it would devastate my boys, he stayed out the other night at his mums, and they were upset. Ive forgiven him so much over the year, hes been physically violent and addicted to drugs. Now hes in full time employment and leads such a quiet life. I cant understand. I feel its so unfair .