I think the best option is "I play viola well for someone having taught himself" (or "herself" if the speaker is female). You can also use "themselves" to be gender-neutral. "Myself" is incorrect because the pronoun refers to "someone", so it should be in the third person. "Themself" is not universally recognized as a valid word, so it should probably be avoided, at least in formal settings.
Here's a citing from Webster's Third New International Dictionary, Unabridged showing the "himself/herself" sense of "themselves":
3: himself, herself — used with a singular antecedent that is
indefinite or that does not specify gender