Do you consider fizzbuzz trivial?

Yes

Could you write an interpreter for a simple Forth-like language, if you wanted to?

Probably; I made a simple lambda-calculus interpret once and started working on a Lisp parser (I don't think I got much further than the 'parsing' bit). Forth looks relatively simple, though correctly parsing quotes and comments is always a bit tricky.

Of course, I don't think I'm a horribly incompetent programmer -- like most humans, I have a high opinion of myself :D