Yea it is indeed very difficult to get people out of that doctrine. Very sad.
Sad?
To believe that the Father is God; that Jesus, his Son, became flesh and walked among us - so that God was walking in our shoes. That we were sinners, godless people, separated from our Creator with no way back; the price we had to pay for our sin was very great and we were spiritually bankrupt, so Jesus offered his own, spotless, life instead. God himself paid the price for our sin, so that we could be clean, forgiven, set free and reconciled to him again. That if we believe this, confess our sins and accept Jesus, we can have God's Holy Spirit - God himself - living inside us; and if we are in Christ, we have every spiritual blessing as well. That one day, when we have finished with this, temporary, decaying, mortal body we will die, but the spiritual life, that has already started in us will continue and we will be with God, in his presence forever?
I think you'll find it's called the Gospel, and is very far from sad; it's the most wonderful, life-changing news ever. I'm not saying that you don't believe the essence of the Gospel, but how does it work without the Trinity? We don't have 3 gods.
If Jesus wasn't God, then it was only a man who died on the cross for us - so how do we know he was dying for us, for our sins, and not his own? If Jesus was only a righteous man and a great teacher, what of his claims that he was one with the Father and that he was crucified for blasphemy, for using the name of God? If Jesus was just a man, or an angel or some kind of spirit, then we have no mediator and no one to intercede for us when we pray - in fact, why even pray? If God is sitting in heaven, untouched by human suffering, indifferent to our needs and only knowing in some theoretical way what it means to be human, how can he help us when we are suffering and being tempted? How can he understand or know what it is like?
If the Spirit isn't God, then the prophets were all lying when they called him "the Spirit of the Lord", as was Jesus when he said that he would send HIS Spirit to live in us. If the Spirit isn't divine and eternal, then he, not it, is created and mortal. Which means that he has no power to tell, or reveal to us, the things of God, or assure us that we are children of God, or enable us to be born again and receive the new birth and new life that Jesus said is necessary to enter the Kingdom of God. It means he hasn't the power to transform us into Jesus' image and likeness (2 Corinthians 3:18), and cannot be the deposit which guarantees our future spiritual inheritance, as taught by Paul, (2 Corinthians 1:22, 5:5; Ephesians 1:14.) It means he can't produce godly fruit, or qualities, in us.
God did not create the world, and us in his image, and then leave us to get on with it; He, himself, took on flesh, was born on earth, in Jesus. He Himself experienced all our temptations, hardships, loneliness, betrayals and sufferings and that we were lost in our sin. He did not come among us with a big whip saying "work harder, be better and you might get to God," he came knowing that we would never be perfect and reach God on our own, so he laid down his life for us. Because of Jesus, we can now be "ransomed , healed, restored, forgiven" and have God himself living IN us.
I don't call that sad - if it was, then why would Jesus have told us to go into the world and take that Good News to other nations?
The Father, the Son and the Spirit are all divine, all equal, all eternal, and there is only ONE God? How is that possible unless all 3 are one?