“I said.” The Hebrew word is “said,” although many commentators assume that God is talking to Himself, and thus translate the phrase as “I thought.” While that may be possible, and in some contexts the Hebrew “said” can mean “said to myself,” it is also possible that God actually “said” something about this situation to His council of spirits.
[For more on God’s council of spirits, see commentary on Gen. 1:26.]
“she will return to me.” This would be a logical assumption. Sin has consequences, and wise people learn from their mistakes. The cruelty and consequences of sexual sin that is part of most ancient idol worship, along with the lack of the blessings of God the people experienced in day-to-day life, should have been enough consequence that the people of Israel and Judah would have come to their senses and returned to God, but they were hard-headed (“the forehead of a prostitute,” Jer. 3:3) and would not return.
This verse in Jeremiah shows that people have the freewill ability to disobey God and do what they want to in spite of His will and desire. God is not in control of what people do on earth.
[For more on God not being in control of what happens on this earth, see commentary on Luke 4:6.]