Hi Stass, well it's good that you have a good heart, but i think i know why you can't forgive him and it's really in what you wrote
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YOU ARE TRYING!!!!! Our human efforts can achieve things, but when it comes to the HEART of God's commands and what Christ taught, when it comes to AGHAPY LOVE. Forget it!!!. You need to REMEMBER and MEDITATE by FAITH in the CROSS OF CHRIST!!. I'ms aying this cheerfully and not being angry at all btw. Just remember who you are (a sinner who has nothing to offer God and yet Christ forgave and died for you). Look to his love, Pray in the Holy Spirit and do it for HIS glory. He loves you and will help you.
read fast Jude 1:21. God give you strength. Also remember than when you do forgive, you would pass a very hard trial and that will defenatly strengthen you espicially if persecution for the word comes.