Selective mutism is a tough one. It usually co-exists with social anxiety and shyness and can lead to depression.

One of the therapies that works with an adult -- you are an adult at your age -- is desensitization therapy. This is something that CBT therapists know about.

For example, you might start slowly, doing things you can do, like communicating through email, hand-written notes, texts and so on. Saying the things you'd like to say to your T. If there's anyone you can speak to, say your mother, you practice speaking to her and then take your mother to therapy with you where you might whisper something to her and she repeats it to your therapist and slowly you begin whispering a few words to your T yourself. Then more words. Small steps forward.

Slowly, over time, the limits of your comfort get pushed as you're able to communicate more.

There are many more desensitization techniques than the ones I've mentioned above. It takes someone specifically trained in the techniques to do it right, otherwise they might move too fast. It's not easy, but it can work.

Your current therapist and a therapist trained specifically in desensitization techniques could work together on a treatment plan for you. Many university psychology departments would be able to provide referrals to therapists qualified in these techniques.

I used desensitization on two problems I had -- an excessive startle response due to PTSD and a simple phobia I'd had since childhood. It improved me about 90% on both. I still have an exaggerated startle response, but I haven't clobbered anyone or even shouted at anyone who came up quietly behind me in years ...and that's a BIG improvement. Desensitization is hard work, but it can bring about big improvements in a relatively short period of time.

It's possible that if you used desensitization to make it possible for you to talk to your current T that s/he would then be able to help you more deeply with anxiety, shyness, depression and whatever else is important for your well being.

I wish you the best.

Edited to add: Just because you're being transferred to adult services soon, it doesn't mean that you need to wait. If you could start desensitization soon, you could have significant improvement by the time your birthday arrives. My phobia was drastically improved in less than a month. I went from paralysis when exposed to phobic conditions to being able to function without fear most of the time when confronted by the formerly feared thing. Good luck to you.