If I have identified the trigger that caused this dream correctly, this dream is about a relationship that you have with someone.
The dream indicates that you did not consider this relationship seriously. A neckless, in a spiritual sense, speaks of the following: A bracelet symbolizes betrothal or a pledge. We are betrothed to Jesus Christ. That speaks of his commitment to the church. The bracelet is close to the heart which speaks of the joining of two hearts. He will never leave us or forsake us. It is important when it comes to faith issues that we can stand on God’s promises. Whatever God said He will do, He definitely will do. This is how faith works. God promised that He will do exactly what his word says. He pledged that if we act a certain way or if we are obedient, He will act and his word will be active in our lives. The bracelet speaks of us belonging to Christ but also us, as the bride of Christ, acting on his word in faith because of his promises. It speaks of the covenant between us and God.
In your case, it is not spiritual. It is about your lack of commitment, pledge or unwillingness to be betrothed to this person. You threw the bracelet away. It had no value in your heart.
The fact that you lost it and another girl picking it up speaks of you losing the opportunity to be betrothed to this guy. You actually lost him because of your lack of commitment.
If this dream is about a relationship that you do not regard seriously enough, the indication is there that you may regret it. You may lose this person. To me, it appears to be a warning.
Of course, if I did identify the trigger incorrectly then this interpretation is wrong. Are you in such a relationship with someone right now? If it is spiritual, then this person may be Christ Jesus, which indicates that you are reckless regarding your spiritual relationship with Christ. I do not know. I think you might know the answer to this question.