"Don't bore me with God had to forsake Jesus or he would not die. With god nothing is impossible."
Saying "had to" or "otherwise this wouldn't have happened" does not mean God could not have done it differently, BUT that he wanted them to happen a certain way and had they gone differently, things would not have happened just the way they did. Therefore in a sense, they "had to" happen that way.
So you may be taking their meaning of "had to" a bit too literally. I don't believe they are trying to strip God of his ability to do something.. merely that it was not part of his plan.. so it had to happen a certain way, like it did.. for his plan to happen.
SO, saying that God "had to" look away from Jesus at the moment he took on the sins of the world and died for them, is NOT saying that it was impossible for God to do it any other way, but merely that for things to happen the WAY IN WHICH God planned them to happen, he "had to" look away.. separate himself from Jesus temporarily until the sins were atoned for.
He COULD Have done it many different ways.. but he planned on it happening the way it did.. so in that context Jesus "had to" be separated from God while he atoned for our sins.
Make some sense?
Now WHY God CHOSE to atone for mankind's separation and sin in that way.. is an entirely different question.
SO.. to answer your PRIMARY question..
"How do you explain that Jesus said from the cross "my god, my god, why hast thou forsaken me"?"
I explain it as "God looking aware, separating himself from Jesus at the moment that Jesus took on the sins of the world and died. Jesus had never been COMPLETELY separated from God, (after all even we aren't) therefore it could have been extremely painful to be cut-off from God.
Jesus's words actually fit Nicely with the Catholic view of hell.. and that is, of a place DEVOID of God's presence. Cut-off from God, completely and utterly.
Early Christian Creeds from the 2nd and 3rd centuries often stated that Jesus "descended into hell" -- which coincides with Jesus's words on the cross, that is "my god, my god, why hast thou forsaken me"? is exactly WHEN Jesus is experiencing this hell.. this utter separation from God, so that Jesus can complete the work of atoning for sins.
Again, WHY it had to happen just this way, and not some other way.. is a totally different question.
I'm just trying to explain the theological view behind Jesus's statement.