Anxiety about volunteering

I have social anxiety but I volunteered to help out a charity tomorrow as its a cause that means a lot to me. My boyfriend was going to come along but now has to work so I will have to go alone.

Now I'm freaking out a bit and really don't want to go. But they already struggle to find enough volunteers so I don't want to let them down.

If I drag my boyfriend along he will have only had 4 hours sleep and have to work straight after so its too unfair to ask him.

I don't know what to do. I have nobody else to ask.