What is mutisim?

How do you cure selective mutisim?

I am in 7th and I have selective mutisim. My mom has taken me to therapy and several differtent people and it still hasn't helped. So some sicoalagist (idk how to spell that sorry) put me on some medicine and it didn't help and made my grades drop. After that we went to my doctor and he gave me a different medicine and thats what im taking now but its not helping that much. It makes me feel more relaxed and calmer instead of jittery and nervous but I still don't feel comfortable enough to talk. I have talked to my mom but she didn't help much she just said "well give it some time" or "Keep trying" but I have. Its really irritating because I really wanna talk but i just don't feel comfortable to actually do it. So could anybody help? Please?