I've been struggling hard with this as well lately, and it's not the first time. I'm prone to depression and being a homebody anyway so the flatline feels hit me pretty bad and I have a real tough time getting out and doing stuff I enjoy. I looked forward to having the freedom away from work and responsibility for so long and now that it's here I've done nearly nothing and it's really not cool. Didn't go out on Halloween, haven't been keeping my place clean, been eating microwave crap even though I love to cook and have mostly snoozed thru my favorite hunting seasons. Feels awful...
I hate to say it but the only way to fix this is to break the habit; the longer you do this the worse it will get.
Practice some tough love on yourself and start small: Go out and just take walks. Little ones at first. Stop and pet some cute dogs. Go out to a bar and just have one drink, maybe work up to making some small talk with the bartender or someone. I generally hate doing that but you'll find that getting out and having a drink and shooting the shit with a stranger about not feeling so great will perk you up some...not saying go discuss the horrors of porn addiction with a complete stranger but just get out and yak a little; literally everyone else has their down days and personal demons they don't bring up and it's honestly real cathartic and goes a long way in not feeling so alone. You might even make a friend or get invited to a cool event.
Little bit of a soap box here but as much as I sometimes criticize our older generations for certain things, we can learn a lot from the way folks dealt with what was bugging them before technology took over. I always meet older guys who clear their head going on walks, out for a beer, sitting on the porch with a pipe, going fishing, sitting in the park and feeding ducks, going out for a country drive, etc. It seems odd these days but seriously, give it a solid chance. Sitting in front of a screen all day ain't doing you any favors as you already know.
Also very important: If you can try hitting the gym or going for a jog when you start feeling more comfortable as this will go a long way into making you feel better.