This doesn't have to be the case. There are many people who also have black dog and are managing to keep the mutt in place. Talk to a counselor who is trained to listen to you and offer the kind of wisdom to keep the black dog from overwhelming you. Let family and close friends in on what you're dealing with. Exercise and diet are proven to help.
And as hard as it might be, stick close to God. The black dog wants you all to itself. So, as hard as it might be, get up and go to church. Sing the songs. Listen to the Scriptures. Hear the pastor as he proclaims the hope of Jesus. Read your Bible each day and pray continously. I know that so much of that sounds impossible. The black dog saps your energy to the point so that you might not even want to get up and get going. Try anyway. Don't make it easy for the black dog to hound you into believing that your life has no purpose or hope.
My dear, sweet wife, Sherri, has loved me unconditionally, along with my kids and grandkids. The congregation I serve, Life in Christ in Peoria, AZ, has been incredibly supportive and helpful. This morning I found a card that was pushed underneath my door. The words come from Isaiah 40:29 - He (God) will give strength to the weary and increases the power of the weak.
Best of all, God is "ever-present" and is ready to help us in any trouble, especially when being followed by a mongrel. Flee to the One who is your refuge, your fortress of protection when depression has left you lonely and afraid. He says, "Come to me, all you who are weary and burdened, and I will give you rest." (Matthew 11:28) You'll receive no better invitation than that.