Makes sense, but wasn't there a bit of a Catch 22? How did you meet your long term partner and develop intimacy, without first improving those skills in general?
My experience is that intimate partners are easier than friends, because whatever awkwardness or other social issues you have, the other person is going to spot it and not go on a second or third date with you unless they can tolerate it. So if you're on a third date, they've already decided that it's provisionally okay with them. Also, I feel like people are way more tolerant of weirdness one-on-one than in groups, because in groups they have to worry about what other people in the group think.
Once you have a partner, or just someone you're dating, they will expose you to other people in social settings, which will give you more practice under a critical and supportive eye.
Just reach out to at least 1 person every single day, whether online or in-person, as a habit. It is an annoying process but pays dividends once you find someone who you like and who wants to be with you.
Once you have a partner, or just someone you're dating, they will expose you to other people in social settings, which will give you more practice under a critical and supportive eye.
But that might just be my experience.