I'm so sorry you are going through this. Like the above member commented, I'm not too sure if you are genuinely asking for advice here, or wanting to vent. If you don't want advice, then, ignore this bit...
You yourself can seem to recognise that your current behaviour is making you feel worse- eg. sitting in a darkened room, isolating yourself. The answer is obvious but the action will be difficult I imagine... Don't do those things! Maybe, after dinner and the dog walk, ask if you can spend some time with your parents.
One of my childhood friends once pointed out to me that our social lives take as much work as our career lives. Friendships take effort and risk to both create and sustain. You could maybe look into joining clubs, doing sports, doing a short course to meet more people with similar interests.
It may all sound horrific but sometimes, often actually, I think we have to make ourselves do uncomfortable things in lIfe for the greater good. If you end up getting nothing but more misery out of trying those things then- at least you can give up knowing you've at least tried to change your situation.
If it's social anxiety or, distrust of others you suffer with, I can definitely relate to that. In truth, I eventually rejected even trying to be sociable but then- it isn't that part of my life that troubles me. This obviously is having a big impact on you.
You could of course find that people aren't reliable and, the whole thing wasn't worth the effort/ discomfort. You won't know until you try though. I suppose, I also think- if a thing still feels worthwhile, it's worth trying multiple times to get it. That's not meaning to make it sound easy though. I know ot won't be.
It's interesting you say you wish you could go back in time to change things. What would you do differently? What makes it impossible to achieve those things now?
Obviously, you know your situation better than I do. You can likely make a better decision on whether it truly is a lost cause. I'm not meaning to belittle your problems here. I suppose all of us have to make an analysis on: How big and bad our problems are. What we could do to address them. If we have the motivation and energy to do that.