Advice needed

I need some advice on a messy situation.

I met someone when I was pregnant with my son, he stayed from the beginning and even though we didn’t have the best relationship I really tried to make it work. He wasn’t on the birth certificate because he’s not his bio dad but was with me for most the pregnancy, birth, and for the first 6 months. We split up when he was 6 months but have gone back and forth trying to make it work and me trying to give my son the family he deserves. Fast forward, my son is a year, I have been trying to make the relationship work but it just isn’t right for me anymore and I just can’t keep doing it when my heart isn’t in it. My family loves me partner and praises him for all he does for me and my son. My son adores him and is truly like his dad since he’s been involved since day 1. We haven’t been living together for a couple months so have been sharing custody and he loves spending time with him and I can see the bond there.

Sharing custody is hard and sometimes I find myself resentful because he is my son but how involved my partner has been I would never say that to him. When I end this relationship, what do I do in regards to my son? My partner has been involved for a year and they have a great bond, do we just coparent indefinitely? Do I cut ties and take away someone from my sons life? I don’t want to end up resentful sharing custody with someone who isn’t related to my son which sounds awful

I just don’t know how to move forward.