The Church Fathers who expanded upon the nature of the Trinity seemed to do a good job. I can’t outdo them, but luckily many of their writings are still with us, so we can still learn from them.
You misunderstand, sir. Christians of all denominations hold that the complete nature of God cannot be known to us as humans. So the Trinity said to be a “mystery”, not because we cannot explain it at all, but because God by His very nature is unknowable to us here on earth. Doesn’t the Quran say about God that "“No vision can grasp Him, but His grasp is over all vision. He is above all comprehension, yet is acquainted with all things” (Qur’an 6:103)? So why are you asking about explaining God, or getting upset when Christians admit that we cannot truly grasp Him in His complete nature?
To my understanding, God is praying because Jesus is fully God AND fully man in the hypostatic union. Unless you are talking to non-Chalcedonian Christians (for example, Ethiopian Orthodox Tewahido Christians), this is probably the answer you will get. What does it mean for Jesus to be fully God and fully man? Well, for the purposes of our discussion, it means He is tempted. This is why He prays to His Father, who is God the Father (the first of the Trinity), for strength to resist His temptations, since “the spirit is willing, but the flesh is weak.”