I have been practicing applying the following verse whenever I have intruding thoughts:
2 Corinthians 10:5
Casting down imaginations, and every high thing that exalts itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ;
Please also take a moment to read a section of Barnes’ Notes, a commentary on this verse:
And every high thing … - Every exalted opinion respecting the dignity and purity of human nature; all the pride of the human heart and of the understanding. All this is opposed to the knowledge of God, and all exalts itself into a vain self-confidence. People entertain vain and unfounded opinions respecting their own excellency, and they feel that they do not need the provisions of the gospel and are unwilling to submit to God.
In response to “letting go of your past”, I am assuming that the struggle you have to let go is because you may have memory flashbacks that remind you of the event or the emotions you felt about it. (Please correct me if I am wrong.)
I think, even after we have asked for forgiveness &/or forgave those that may have hurt us, these memories can keep us from fully receiving God’s mercy and grace. In a sense, it is the enemy that continually rekindles the pride of the flesh in us, as those thoughts/memories try to get us to think God is not in control and that He doesn’t know what He is doing.
Each and every time my thoughts take me away from focusing on Jesus (i.e., unhealthy thoughts), I say to myself that “I take these captive to You Jesus, in obedience, because I trust You.” Sometimes it seems like I say it a thousand times a day, but I’ve encountered that it helps me refocus and remember who is in charge of me, Christ Jesus.
Hope this helps a little