He sees no future with me

Four years ago my husband died from Covid and a friend of mine ( we were friends for over 20 years) helped me and came to my side. He broke up with his life partner and I was widowed and we began a relationship that turned into a situation ship. About 2 years in he told me he wants to continue living with his ex as a co parent but there was nothing going on ( they have two kids together) I was fine with that - last week he says he does not see a future with me he wants to devote his time and life to his kids. Not sure how to keep the friendship and move on. And never will be able to understand why someone I knew and trusted would sleep with me after my husband died then go back to his ex. I know I have to move on but it isn’t easy.