How to love him still but let go of the old?

My ex and I have been together for almost 4 years. I began to have bad anxiety after year 1-2 due to school and I put him through a lot of stress. On top of dealing with my anxiety, he also had to deal with his own mental health and the extra anxiety his mom goes through too, and his mom has it worse than I do, unfortunately. So its a lot for him to deal with on a weekly basis. 4 months ago we decided to take a break and I work on my anxiety and depression. While on the break he started seeing someone while still having feelings for me. They got intimate. And he told me the truth sometime within those 4 months. I gave him the chance to figure things out if he stops what hes doing. 2 days ago he confessed to me that hes still doing what hes doing. Reasons for his poor decision making was due to giving into temptation and his childhood trauma. I believe when he says he doesnt want to be that person since he broke the hearts of 2 women this weekend.

We decided to end things and give each other space to heal because we still consider one another as best friends. Were both Christian and lean on the faith that God will lead us on what to do based on what the Holy Spirit tells us. By the time I talk to him again (in a long time from now) we both will be different people. How should I move on and heal while still loving him as my best friend? I want to let go of our old relationship, the old me, the old him. How do I let go, forgive and grow?