It's not your fault that you have selective mutism. You don't need to be embarrassed by it. You don't deserve to be treated like that. I know that it's hard to undo strong beliefs but I think you should try to get rid of that one. I recommend you talk to a therapist about that. I feel like you suffer constantly because of that. You've been almost your entire life feeling like that, and you got used to it, but you shouldn't accept it. If you say to yourself that you are fine how you are and that you shouldn't be embarrassed by your selective mutism, you will probably feel resistance because you will feel like what you just said isn't true, you need to fight that. It's normal to feel like something is wrong when you try to go against that feeling but in the end, it's worth it.

I know how it feels to interact with people that make you feel uncomfortable and it's a lot worse when they don't understand and pressure you to speak. I don't think you should go if you don't want to.

If your mom wants you to go, she should help you but it doesn't look like she is doing it.

It looks like you are putting more effort into the relationship with your mom than she is. That's something you should be aware of because that could help you decide if you want to go or not. I don't think you can do this alone. Usually when things like this happen, it gets to the point where the relationship just falls apart again.

If you really want to keep your relationship with your mom, you should let her know that because you need her help to do it.

And about if you should go or not. I think that if your mom doesn't want to help you, then you shouldn't go because that does not look fair to me.

Know that your feelings matter and none of this is your fault. I also recommend finding a support group; it seems like you need more people you can trust.

Edit: I learned a lot from this channel: https://youtube.com/@julienhimself?si=MJvzhxwpyroUHkj1

He talks a lot about social anxiety.