My inner demon

Ever since I woke in my bathroom, covered in cuts and blood, a voice has returned in my mind. A voice I haven't heard since I was a child. Since it's return, I have constantly blacked out, causing me to wake up in peculiar places, in indescribable pain and confusion. He says he's Dr Umbra but I know there's more to him.. or more to 'it'. I'm too afraid to tell people about him, for the risk of being called crazy and unstable is too high. But he is driving me over the edge: both physically and mentally. Is there any way for him to leave me for good? or am I trapped with him forever?