It's not easy to say whether it is or not. This is something called the Liar Paradox and it has a surprising amount of potential solutions. That article linked explains it really well but, be warned, it is a bit dry.

The solution one of my professors gave that makes most sense to me is that, as a standalone sentence, "this sentence is a lie" is neither true nor false. At first glance the sentence makes sense and "lie" leads us to think that there is an untruth somewhere but there can't be as there is no 'truth value' within it. That is to say that there is nothing in the sentence that can either be true or false therefore there is nothing that can be lied about.

Only one of many potential solutions so though. So, maybe?

"You can't unshit yourself".

A bit crass maybe but it's always stuck with me. Basically a way of thinking in bad situations that need action eg if you've shat yourself, no amount of wishing you hadn't is going to change the fact that you've shat yourself, so what are you going to actually do about it?

Hopefully it's metaphorical but works on both counts.