This has been going on since you were 16? No wonder you’re getting burned out. At 21, you shouldn’t have to be your father’s keeper. You should be launching yourself into your own adult life. Your father apparently has no idea how he is keeping both of you stuck.
I think you’re right that your dad is depressed. He’s self-medicating with overwork and alcohol. Neither remedy is effective. In fact, they only are making things worse.
I can only suggest a few things: First, instead of being angry with him or trying to reason with him, simply tell him how worried you are about him and how his depression is affecting you. Don’t do this in a blaming or shaming way. Just tell him you love him and are feeling unable to make life decisions because you don’t want to leave him when he’s feeling so low.
Then, encourage him to get into some therapy to deal with his feelings of grief and anger about the divorce. Tell him you will go with him for awhile because you need help finding ways to be supportive of him. I’m hoping that his concerns for you as a father may matter enough that he’ll do this for you even if he won’t do it for himself. Hopefully, your therapist will be able to engage him so that you can turn his treatment over to her or him.
Finally, I strongly urge you to find a local chapter of Al-Anon. This is an organization that is connected to Alcoholics Anonymous and that provides support for family and friends of alcoholics. The program can teach you skills to help both your father and yourself. Most important, the group can give you needed support.
I wish you well.