Navigating Mixed Feelings About Being Friends After a Failed Relationship

How do I tell if I actually want to be friends with someone or not? I was dating this guy at college for a couple months, but then a month or two before summer break hit we realized we would have to be long distance over the break. I was willing to put in the effort, but he told me he would be busy over the summer and a relationship wasn't doable, so he suggested we be friends for the summer and then think about dating after summer. I agreed, we continued flirting as friends instead of as partners for the remainder of school, then over summer it stopped. Fast forward, this summer we've only talked if I start the convo, and never for long. I missed him, but he's busy and has ADHD and always sucked at texting, so I figured that was why. Turns out he found someone else over the summer and he wants to be only friends going forward. I want to be his friend because he's a good friend to me, but I'm honestly just really mad from how he treated me as a partner. I don't think he ever cared about me the way I cared about him. I can't say he led me on because I knew that him meeting someone else was a possibility. He never once lied to me, I just was dumb to agree because I thought he still had feelings and intended to be with me when he proposed the idea (turns out he didn't, he just wanted to keep options open). I want to be angry, but I can't because I agreed to this situation. Now that the rose colored glasses have fallen off, I'm realizing that even when we were dating he neglected me a lot and didn't treat me how he should have. Since he broke the news to me, I've asked for some space. I want to be his friend, but should I? I feel so upset right now, but it feels redundant to bring up how bad of a partner he was when we're just friends now and were already "friends" before school even let out. I want to be his friend because he was a good friend to me, but I don't know if I can get over feeling angry at how bad of a partner he was. I'm thinking I should be grateful we aren't dating and that since he is a good friend to me, I should be his friend (after taking some space to get over feeling upset). Does that sound like a good idea or do I actually have a right to feel upset?