How do I handle him moving on after 1 month?

I (29f) and my boyfriend (28m) of 3 years decided to break up due to our differences. We are still separating finances and he agreed to keep me on his phone bill. I co-signed a car and he pays it with the rent as I left the apartment and I pay insurance and the phone bill. I’m at my moms now He said he cares about me and still sees me as family and will always be there for me but I found out through Facebook he made a new “friend”. He said she’s fine with us being friends. Just hearing about her in passing upset me. He said it’s not serious and he finds it weird that I’m mad he moved on. Today on Thanksgiving he told me he’s going to spend time with her and I felt a type of way. I decided to tell him we can’t be friends because this is all so fresh for me. He said it’s okay and he’s thankful for everything I did. I feel angry because I feel like I didn’t matter at all. I did so much for him and he just finds a new girl just like that. After years of me begging to work on us and go to therapy he’s going to do everything I needed for some other girl. I’m alone with no real friends and am not ready to get out there. I’m going to volunteer in my extracurriculars soon so I’m gonna keep myself busy. I’m just worried about how to focus on myself. What do I do after I cut him off?

How do I handle him moving on after 1 month?
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