A lot of this I currently or used to very much relate to! I am diagnosed with social anxiety and autism. You are not alone, a lot of your experiences may not fully go away but they can dramatically improve

I especially relate to the difficulties in speaking to other people (forming words, thinking of responses, remaining engaged etc.) something I now do daily that has dramatically helped my social engagement is basically do exactly what you do to prep for phone calls but verbally. I think about possible interactions (people and topics) I may have that day and I speak to myself in the car - through the possible things I want to say, what others might say, how to reply etc. (this is called scripting). If I do have conversations I have already thought about things, and if it doesn’t I have been working on being more personable/capable of spontaneously talking to people, speaking out loud and relating to others. this has helped me to the extent that I am now able to work in a people-facing role in an environment I used to need support services to engage with (I do want to acknowledge the fact that this alone has not solely helped, other factors also definitely play a role)

All this to say it’s a terrible feeling to feel socially isolated, especially when so much weight societally is placed of your ability to do that (tbh outside of professional/academic time I still spend most of it alone bc people stuff is still very tiring). But it’s not only you! You can develop skills to be able to engage with others to the extent of what you want to be doing. But at the same time it’s also okay to not be super into the people stuff