AITA for trying to tell someone about their best friends faults?

So I (14f), know these two people. One we'll call Sam (15m) and the other we'll call Don (15m). Sam and I dated a while back, but we broke up on good terms, and stayed friends. Don and I got together a little while after. The main complication here is that Don and Sam are super close friends. I broke up with Don because someone came up to me and showed me text screenshots of Don telling her he was going to cheat on me with her, dump me for her, ect. I asked Don about it and he confirmed, also telling me that he only got with me because he felt bad for me, but would have rather been with this other girl. All this made me feel like shit, so I dumped him. I was still in love with Sam, and since we're in school, on-and-off dating is pretty common. So I asked out Sam and he said no. I noticed afterwards that Sam was being very cold and ignorant towards me, so I asked if he was mad at me. He said yes, because I "broke up with his best friend to get with him" I told him that wasn't the reason and Sam asked what the reason was. I told him about what Don did and Sam wouldn't believe it, saying "Don's my best friend, he would never do that." I tried to tell him about how Don literally admitted it, and Sam just wouldn't have it. He didn't even give me the slightest chance. I admire his loyalty, but not when it's to the point that he believes Don has no flaws. I told him to just ask Don and he wouldn't even do that, which sort of says that he was afraid of the answer. If he truly thought Don wouldn't do that, he wouldn't be afraid to ask, right? Anyways, Sam has now blocked me and Don isn't talking to me either. I feel terrible. Sam was into me for two years before all this, and since we didn't talk very often, he was bound to make up fantasies of what kind of person I was in his head. So I feel like I've disappointed him with all of this. Sam and Don are both really good, kind people and they helped me into a big group of amazing people, now they all are at least mildly upset with me. I feel terrible about all of this. AITA?