How do you explain Matthew 26:39 Jesus wanted this “cup” (his death) to pass from him,  but could not do it..?

Again, this is a limitation that Christ placed on himself. Omnipotence is not the ability to do anything, but the power to do anything in keeping with one’s nature and will (e.g., God cannot lie; God cannot build a rock to large to lift, etc.). Once God willed to save men, there was no other way for God to do this except through the death of Christ. Jesus did not relish the prospect of death and wished there was another way to perform the will of the Father, but it could be no other way. Christ had obliged himself to die when he chose to save us. Again, it could be no other way because Christ would have it no other way.