If Jesus is God, then why in John 8:28 did he say “I do nothing on my own authority, but speak just as the Father taught me. And he who sent me is with me. He has not left me alone, for I always do the things that are pleasing to him.”? Can’t God do anything he wants?

Jesus was fulfilling his ministry in its entirety as a man. Galatians 4:4 tells us that Jesus was born and operated under the mosaic law. As such, he was subject to all of it’s ordinances, such as circumcision (Gen. 17:10) and not eating pork (Lev. 11:7); and he fulfilled his ministry by emptying himself in servitude even to the point of death on a cross (Phil. 2:5-8). Since he was under obligation to uphold the law (which required complete dependence upon the Father), and since he had emptied himself and was working in cooperation with limitations, he would naturally do nothing of himself.