I’m going to be the same 20 years from now; alone, single, and miserable.

See your self-talk?
It reminds me of how fear driven my own self-talk can be.
I think single people tend to idealise all those in relationships.
I think I’m happier in a relationship now but we certainly have our moments, our arguments, our times when we ask ourselves” is this what I really want”>
And then some day one of us will die anyway. We have to get used to change. Nothing lasts forever.
You know I look back at my single life and sure it was lonely sometimes but on the other hand I had a kind of independence too. It wasn’t all bad. It was an interesting phase of my life.
Try to have gratitude for your current phase of life, I guess is what I’m trying to say.