We are a new creature because we have been raised from death to life instantly at salvation, but that isn't the birth of the spiritual man.
The Spirit gives life to spirit, John 3:6.
When we are saved we are raised from death to life, yes; spiritual, eternal life. The Spirit gives that life.
Paul said that we are to be filled with the Spirit, Ephesians 5:18, that we are to walk in the Spirit, Galatians 5:16, that we have been set free by the Spirit, Romans 8:2, that we live by the Spirit, Romans 8:4-5, that we are not to grieve the Spirit, Ephesians 4:30, that we have taken off our old self and put on our new self, Colossians 3:9-10, that we are new creations, 2 Corinthians 5:17, that our new life is hidden with Christ in God, Colossians 3:3 - and I'm sure there are many other references to our new life and the work that the Spirit does in us.
How could any of this happen without us being spiritually reborn?
The spiritual man, or Christ in us, is something that comes later. It has to be worked for.
None of us can ever work to earn any gift or blessings that God has for us.
If we could, then we would be able to boast that our spiritual life was our doing and the result of our efforts. That is not so and can never be; all is from God.
In Galatians 4:19 Paul is travailing AGAIN... meaning this isn't the first time he has been laboring to birth Christ in the SAVED Galatians.
Yes, he was preaching Christ to them again - because they had been listening to a false Gospel, Galatians 1:6-9. They had been listening to false teachers who said that Christ was not enough and they need to be circumcised to be saved. It seems that those teachers were in, or near, Galatia and were more easily able to tempt the Galatians away - Paul was elsewhere and had heard about what was going on. So he had to persuade them, once again, that the Gospel was true, NOT what these teachers were saying.
Paul said that such teachers should be condemned to hell, and then went on to say that if the Galatian Christians submitted to circumcision for salvation they were saying that Christ meant nothing to them, Galatians 5:2.
It is the Spirit's job to bring people to Christ. No-one can make that happen; all we can do is present the Gospel and pray that the Spirit of truth will lead them into all truth. We are told to instruct one another, teach the truth and lead people away from error. And all of us grow in knowledge and holiness.
But that is not the same as saying that we have to work to receive Christ in us, or be born again.