This is an awful situation for you op 
Why do you say you have brought this on yourself? Depression is an illness and a horrible one at that. Is it because you think you have enabled him too much? Please don't think that. We all want the best for our DC and he was v young and you were going through family troubles when it started. You are being far too hard on yourself. And however hard this feels to have to do, he needs you to model a strong and positive attitude right now, so please don't allow guilt to get in the way of that. You can't change the past, you are where you are at right now.
Is he in his early twenties now? It sounds like he needs a complete change of scene/alternative perspective which could be gained from volunteering or travel. Would he be too depressed to do that currently?
Some sort of eco volunteering project might be good where he builds footpaths or repairs dry stone walls as they provide: structure, a new skill, physical work, companionship, a regular routine, fresh air and exercise, in beautiful surroundings and a sense of achievement, which are all good for mh. It will probably be hard to find something like that ATM but might be worth looking for next spring if not before. It's important he has a goal he can look towards.
I think you need to tell him that you love him too much to enable him to sit and do nothing all day anymore and that he deserves a better life. Seek some support from Young Minds and tell him no more money will be forthcoming if he doesn't get himself to the gp in X amount of days and that you are expecting him to help himself to live independently in nine months to one year's time.
Could you take him out of the house - go on a walk in calm surroundings - and ask him what he wants out of life? What are the things that interest him? What are his talents? Try to get him thinking about his future? (I am sure you will have done this before.)
Is his dad around? Could he go there? Could he help with funding work experience or funding travel, or therapy?
Do you have any family members you could reach out to for support op? This is a very hard thing to do all on your own 
I really feel for you dealing with this on your own op. There isn't enough help out there. If I were PM, I would set up a top notch project to help despairing parents with their teens and young adults who have lost direction in life. There are far too many of them currently. 