Fucked Up

I genuinely fucked up tonight. I had made things good with my ex gf again and things were going great. She mentioned her son and had come to the conclusion on a few things. Of which I agreed. One thing was to help him get his stuff together so to speak. So she asked me if I could help by giving him chores when he got home tonight. I did and he started with attitude to me and his mom. I didn't take kindly to this so I swatted him on the but once with my belt. I had done this before with his mom's permission but he always had jeans on. My first mistake biggest was not letting Mom know I was using a belt. The second was that I ended up leaving a bruise on his behind. He started again and this time I used my bare hand. Just once mind you. But once too many times. I really fucked things up with her because before anything else she is a great Mom and it seemed like I was a good team with her. Well she thinks otherwise and I don't blame her. She asked me to leave and in so many words said we could forget about us.
The thing is I love these kids like my own. I am proud of them and would do anything for them. I just totally fucked things up tonight. I have volunteered to talk to her Dad about it. I have volunteered to talk to anyone she wants me to about it.
I don't know what to do except know that I am going to miss them all. I already do. I have never done anything like this before in my life. I don't know why I did it without asking her first. I do love them and do care so much about them. But I understand where she is coming from. And don't blame her at all. She used to leave her kids in my care while she went out shopping. I loved having them too. I have had run ins with her son but never anything like this.
Like I said I FUCKED UP!! and know there is no way I can get them back.
I will always love them and have a special place in my heart for them. I don't understand why I did this. I never did anything like this with my own son when he was growing up.
I love them so much!