I'm not trying to play doctor but I have some experience with your situation. Severe, chronic depression, as you describe is a medical condition. A doctor may be able to prescribe an anti depressant that will greatly improve your quality of life. No guarantees, but I resisted for many years but finally was persuaded to seek help. An inexpensive pill, once a day has helped me immeasurably.
See your medical doctor who may be able to prescribe something for you or refer you to a psychiatrist who can evaluate your situation and make you aware of your options. If the first thing you try doesn't work for you, try something else, don't give up. There are several different approaches that may not be right for you but its likely one will work.
Good luck.