I've done the exact same thing my entire life. I was waaay worse when I was your age but things got significantly easier once I was put on medication for my anxiety (zoloft). Medication doesn't work with everyone but I definitely saw improvement. I stopped overthinking my social interactions and just began reacting naturally. I'm not perfect now but i am way better off.

There's a book that i found because of this sub called "How to win friends and influence people". There are free pdfs of it online. This book goes through the normal behavior of a likeable person. I use it as a checklist in my social interactions. Mostly to ease my anxieties about handling the situation normally. You can also practice lines in your head for specific social situations. For example, I have a lot of trouble with the greeting "how are you?" because my difficulty with social interactions. So i started making myself practice saying the words, "I'm fine. How are you?"

Since you're 15, I'm sure you're in high school which is one of the worst places to be when you have social anxiety. high school can be literal hell, but the good thing is that most teenagers are too self absorbed to look outside of themselves. Most likely you are the only one who notices your difficulty with social interactions. You might feel like you're alone but I'm sure your school is full of people just like you. Everyone is still just trying to figure out language and how to interact with each other when they're your age. I hope i helped!