IF Jesus is God then whom was he addressing in the following: (verses Matthew 27:45-46, Matthew 26:39, 42”)
Clearly, it was a plea to Adonai, the one true God of the Jews. Jews did not believe that God would come to Earth as a man and Jesus, as a Jew, would have considered such a belief as blasphemy. He also understood that Jewish Messianiac prophecy could not be fulfilled without, at a minimum, “Gather all Jews back to the Land of Israel” (Isaiah 43:5-6), among other things, and the realization that he had failed must have been overwhelming. It’s pretty easy to understand giving the context of the times.
Jesus wasn't just a "Jew", he is the Son of God. He healed on the sabbath; he called God his Father; he call himself the Messiah; he permitted others to call him Messiah; he forgave sins; he allowed the rich young ruler to call him "good", although Jesus said only God is good; one can be saved by believing on God or believing on the Son of man -- take your pick, they're the same God; John the Baptist called him "the Lamb of God which taketh away the sin of the world"; when Jesus comes [again] he will gather his elect (all saints) from the four corners of the earth (see Matthew 24:31, Mark 13:17, Revelation 7:1).
...and the realization that he had failed must have been overwhelming.
Not so fast. The rescue of his elect -- doesn't sound like failure to me.
Behold, I show you a mystery; We shall not all sleep, but we shall all be changed, In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed. For this corruptible must put on incorruption, and this mortal must put on immortality. So when this corruptible shall have put on immortality, then shall be brought to pass the saying that is written, Death is swallowed up in victory. (1 Corinthians 15:54 -- quoting Isaiah 25:8)
Today is the accepted time; today is the day for salvation.
Jesus said unto her, I am the resurrection, and the life: he that believeth in me, though he were dead, yet shall he live: And whosoever liveth and believeth in me shall never die. Believest thou this? (John 11:25,26)