私僕 both mean "I/me" but are used differently. 私: formal way of saying "I/me". Read as "watashi". ("watakushi" in very formal speech.) 僕: informal equivalent of watashi. Used only among friends and only by males. Read as "boku". Use in plain speech without -Masu. (desu also becomes da when boku is used.)

本当ですよね。でも「この僕でいいの？」可哀想ことでしょう。

「高慢」と、「謙虚(humble)」の、どちらでも使えると思います。If you say「この僕が、～～に選ばれたなんて・・・信じられません」, it'd be like "I just can't believe that I (and nobody else) was selected as a ~~ (implies that I hadn't imagined that I would be selected". If your gf says "結婚しましょ" and you say "えっ、この僕でいいの?" then you'd sound like "Oh me, nobody else but me? Are you sure? Do I deserve you?"

あーそうですか。高慢何かみたい。ありがとうございましたチョコレットさん。

「えっ、私/僕(が)?」sounds to me like just "Oh, me?" and 「えっ、この私/僕(が)?」sounds to me like "Oh, nobody else but me?」ーー「（よくも）この俺に向かって」sounds to me like "How dare you say that to me (the important me / nobody else but me.)"