Did I say that Reb Meir said Libi Chalal Bekirbi? I explained that he scoffed at those who gave in to their desires, and that he was, obviously, very removed from that struggle.
The Gemara actually explains that Dovid Hamelech said of himself that he had no Yetzer Hara. His Chet was caused by him asking for a Nisayon, and for himself actually wanting not to be triumphant on Hashem, and to show the power of Teshuva. Therefore, this answers your question about why Reb Meir couldn’t bring proof from David Hamelech that the Yetzer is very powerful. That case was an exception as David Hamelech himself said, that he has no Yetzer Hara.