My little children, of whom I travail in birth again until Christ be formed in you,
Note 12 at Ga 4:19: Through prayer, Paul had interceded for these Galatians when he first brought the Gospel to them. Here, he was in doubt of their relationship to Christ, since they were moving back toward trusting in the Law. Therefore, he was praying for them again as he did in the beginning.
This does not mean that the Galatians had lost their salvation and that Paul was praying for them to be born again, again. That can't happen (see note 5 at Ac 5:5). He was simply stating that he wanted to see the Galatians turn back to Christ and develop and grow in Him again.