A lot of people, including me, can empathise with that. These are the lies you tell yourself when you have anxiety through low self esteem. The over-analysis is typical. Does that sound right to you?
Thinking distortions are really common. It seems to me that you are imagining what people may think and feel and treating those thoughts as reality. No wonder you are dreading it, you are telling yourself it will be crap for you and for those around you and that it will all be your fault. That is one hell of a powerful negative prediction. The over-analysis afterwards serves to magnify and reinforce your negative beliefs about yourself, we can all find something less than perfect about ourselves, especially if we look hard enough. When you have a downer on yourself, you are your own worst critic and things others barely noticed will seem awful to you. Even if no one else notices anything you will tell yourself they were just being polite or that you had a lucky escape. This analysis can then increase fears around socialising.
Any of that sound familiar?
I know telling you it is all in your mind doesn't make the feelings go away. Why not try the evidence based approach?
Get a pen and paper and write down your worst fears about the do. What is the worst that can possibly happen? Give it a percentage for how likely you feel that result is. Next write down what is the best that can possibly happen. Give that a percentage score too.
Next write down facts, not feelings, supporting each view. Do your colleagues tell you you are annoying and boring or are you mind-reading (a common trait)? If you have been told either of those is that just from the office bully? Is it the majority? Is it just you telling yourself that? If someone ends a conversation with you and moves to chat with someone else, are you the only person that happens to or does it happen to everyone? (hint, it happens to everyone
Can you identify any beliefs that may underlay the anxiety? Can you write those down and score those too? Can you list evidence for and against each belief?
An example could be Perfectionism. This is a common anxiety trait. Are you genuinely annoying and boring or are you just setting ridiculously high standards for yourself to be the centre of attention, to gain everyones approval? Not everyone likes everyone and no one demands anyone be the centre of attention. It is ok to be you.
Any of of that make sense to you?
Once you have balanced the evidence, write down versions of what might happen and what you believe based on that evidence. Rate the probabilities.
Finally, re-rate your original fears and beliefs. See if you still believe them.
All that may sound strange but you have nothing to lose by giving it a go and you may gain something.
Hope some of that helps.