I suffered from selective mutism when I was a child and young teenager. There wasn't much help in those days but now I read a lot about treatment
and successes. Were you diagnosed with SM ever, and treated by a specialist in the field? There's an excellent organization, called something like "Selective Mutism Group". You can look it up online. There is a lot information about treating children, but I used to read posts from teenagers and adults too.

SnakeCharmer gave you some good advice as to how selective mutism is treated. I also read that Prozac or similar meds helped with the anxiety.

When I had SM, it was called elective mutism because it was thought that people CHOSE not to speak. I wasn't silent because I wanted to be but because I just couldn't get the words out! I was fortunate that I got over the mutism by myself but I was always shy. Therapy was hard even though I talked by then. Writing is still easier for me than speaking. I would suggest seeing a T who knows something about treating people with SM. I also think an art therapist might work. When I am busy using my hands in an activity (once I fingerpainted) in my session, I found that it was easier to talk. I empathize with you and understand your struggles! I hope you can get the help you need to get better.