Earlier quoted context omitted.
I had a bout of this about 20 years ago, so I made a rule for myself: I wasn't allowed to decline an invitation. If somebody proposed doing something together and I was able, I had to do it. That made a huge difference in my life. I did a lot of things that were... unexpected. (A few examples: I joined an ultimate frisbee league. I went skydiving. I joined a church group. I went to a strip club. I attended a wedding.…
>I had a bout of this about 20 years ago, so I made a rule for myself: I wasn't allowed to decline an invitation. That approach works great as long as you are currently getting invited to things. If it's been a year since the last time you were invited to anything, that's not going to solve your loneliness problem.
But if you have normal interactions with people on a regular basis, you'll get invitations. They might be tiny or tentative or impersonal, but if engage with questions like "Can I talk to you for a second?" or "Would you like a sample of our new Teriyaki Chicken?" or even "Do you have the time?" you'll gradually open yourself up to more possibilities.