He also said something along the lines of "Failed I have, into hiding I must go", meaning he thinks he is no longer worthy of the title of Grand Master of the Jedi Order. He also resisted Obi-Wan's suggestion to train Luke, not because he was "too old", though that is the excuse he used, but also because he did not feel like he could do it because he didn't think himself a Jedi. Also, he didn't say "the last of the Jedi will you be" until Ep. 6. The "Do or do not" was in Ep. 5. Sometime in between he decided that yes, he could still be a Jedi, even though he had given the title up Jedi.
He went into hiding because he
failed to cut the head off the snake. He already knew what happened to the other Jedi, and with Sidious still alive, the clones would continue to pursue him. He didn't want to train Luke because, like his
father, He had aged and formed deep
attachments to people. This played out when Vader tortured Han, specifically to draw Luke out. The Emperor further utilized Luke's longing for a father figure in ROTJ. "His compassion for you shall be his undoing. He will come to you, and then you will bring him before me." Yoda also, in the third person under guise, referred to himself as a Jedi Master. "Jedi Master? Yoda. You seek Yoda?...Take you to him I will. Berheheheh, but first, we must must eat. Come." Yoda was testing Luke. I'm fairly certain that Luke was not yet attuned to sensing the presence of another Force wielder. Yoda strung Luke along to see if he could calm down a sense what was really happening, and that Yoda was right in front of him.
You are also reading lines spoken over the course of only 20 some odd years as differences of mind set of a being that is 900yo. It is highly unlikely that Yoda would lose faith in the core beliefs he has carried for
centuries simply because his enemy gained the upper hand. That would have been like the British giving up on reclaiming the Colonies after we took our first major victory. Yoda simply understood that, at this point in time, it was futile to fight. Although, I do think he gave up rather easy in the Senate, considering what was at stake.
If I'm tired enough :p no problem. But driving around town in pure darkness will lead to allot of death.
Even the shade is a form of darkness. Light and dark have to coexist in order to maintain life.