I had selective mutism at school. I was able to talk to friends in small groups / one to one but was silent in most classes (One school report read "virtually mute").

It carried on into my adult life, mainly because of my choice of office based jobs. An office job feels very much like a classroom to me and I just don't think I ever got over that fear of exposing my vulnerabilities. On top of that, speaking was a trigger for blushing which I used to dread so I realized that no talking meant less blushing.

Years later I still have a problem with speaking in front of others but it's better. I don't join in group banter but can say a few words here and there. I also notice that if I don't prepare I'm ok. If someone walks up unexpectedly and asks me a question I can bat back a reply without much bother. It's just initiating speaking which is tough.

As for making friends, one thing I've learnt is that some people like quiet friends. We make good listeners. So if you like someone just use body language to start with rather than forcing yourself to speak, try and smile (which is easy if you like someone) and throw them the occasional look when they're talking and you may be approached by them.

Take it slow and don't force yourself to be who you think people want you to be - that doesn't work and will always increase anxiety and may even get you friendly with the wrong people. You want to make friends who like you for who you are. And don't despair if you don't make friends straight away - sometimes it takes a while for potential friends to reveal themselves. I've befriended people in the past who I didn't think I'd ever be friends with when I first met them.