@Jesper Welcome to BassBuzz!
When you have time, join us on the Introduce Yourself! thread.
You are not alone. Billie Jean is (arguably) the most difficult piece of music in the course. Most people that have talked about it on the forum didn’t mange to play it at the fast speed until after they had finished the course. That bass line is terribly misleading. It only seems simple.
I’m not sure exactly where your talking about with the muting but I can tell you that muting is a process and it’s something where you just have to use whatever finger is available at the time.
If you can be a little more specific about where you’re trying to mute, I can try to tell you how I play it.