Is Selective Mutism Only Anxiety-Based?

If it’s a choice, then it’s not selective mutism. I physically can’t speak in certain situations. If it’s not a choice, it could be the result of depression or anxiety

Doesn't sound like you have selective mutism.

Selective mutism is an anxiety disorder. It's classified as when someone who otherwise is fully capable of speech cannot speak when exposed to specific triggers (such as places or social situations). It is also extremely related to social anxiety with two studies showing that 100% of people with selective mutism had social anxiety and another study showing 97%.

Definitely not only anxiety based, but that is a major contributor in a majority of cases. Sometimes it happens to be because of depression and I feel like no one wants to hear me so I might as well not say anything.