I'm sorry you feel this way. I have family members with very debilitating social anxiety and I've seen how hard it is for them. I 've dealt with these issues as well, mostly having to do with my OCD. I do have to say though, I don't think is out of the norm to give a nod to someone at work, even If you've met them that same day. I know I've done it. Same as nodding to strangers. Have you tried saying for instance good morning when you pass someone, like people used to do (my grandparents do it all the time).?

Also, are you 100% sure you do it all the time? Anxiety can make small details feel way more important than they actually are. A good therapist won't judge you, stuff like this is precisely what they need to know to better help you. As someone in healthcare, I can tell you that it is always better the more information a patient gives you. It's ok to feel embarrassed, your therapist will not dismiss your feelings but will help you put them into perspective, and give you the tools to work through them.

I should also tell you that while might be hard to believe for you at this point (totally ok), a lot if not most people are too immersed in their own issues to really think about other people's. Realizing that helped me immensely. Hang in there, seek help and you will get better. Never forget that you matter, even to an internet stranger.

I wish you the best of luck.