Adam Clarke on Galatians 4:19

My little children - Τεκνια μου· My beloved children. As their conversion to God had been the fruit of much labor, prayers, and tears, so he felt them as his children, and peculiarly dear to him, because he had been the means of bringing them to the knowledge of the truth; therefore he represents himself as suffering the same anxiety and distress which he endured at first when he preached the Gospel to them, when their conversion to Christianity was a matter of great doubt and uncertainty. The metaphor which he uses needs no explanation.

Until Christ be formed in you - Till you once more receive the Spirit and unction of Christ in your hearts, from which you are fallen, by your rejection of the spirit of the Gospel.