Hi Snufkin! I abolutely get the overthinking and social anxiety you are feeling as it's the same for me. I am currently in rehab in Thailand and have met a wonderful chap called Paul Garrigan who is a mindfulness coach. He centres on the fact that "you are not your thoughts" and is teaching me techniques on how to let the thoughts pass and understand/accept that I don't control them, they are what they are but I can control how I act on those thoughts. If you put into google "paul garrigan mindfulness" you will find his site which has a lot of good stuff on it.
“To see our own mind clearly, without being caught up in its movement, to watch thought without trying to do anything with or about it, simply seeing it and letting it go, this is the way to freedom from suffering”
Luangpor Teean (The One Who Feels)