If truth can be obliterated/crushed because a known liar INSISTS the truth is a lie are we done for? Should we just give up and die?

Nah---Truth exists and is the proper object of study.  And it can and is found by many people.  But of course, there is nothing that we who have discovered the truth can say to compel your another's belief in it.   But of course, that's not our problem---we are happy to share it, but it's on a "take it or leave it basis."