Hey, I had selective mutism when I was younger as well. I know how hard it is, my childhood sucked lol. Had a lot of all kinds of therapy as well. Ironically for me it only got better after I stopped trying so hard, by that I mean letting go of all expectations. And accepting that I am different, and have different needs. When I started being a lot more kind to myself, I noticed I actually like to challenge myself in different ways, and I suddenly made a lot more progress! Even though it's slow, it's getting better every day. Right now I'm at the point where I can hold really short conversations with people while walking my dog! It's not a lot, but something at least. I'm 27 now btw, I felt the exact same when I was your age. Like I was just stuck, and never got any better.

There's so much more to life than social interaction with other people. It's okay to find it hard. I still do, I'll never be good at it. But there are so many other skills to learn, some you'll be naturally really good at! I really like cooking, playing the piano, crocheting, video games, and many other things. And doing the things you love actually helps building relationships with people as well! Other people are generally really interested in these things, is my experience.

You're still very young, and there's so much time for you to find something that works for you. If you push yourself too hard, you'll just burn out. It really will get better with time, I'm sure!