My ex is very hurt that I'm moving on, I don't know what to tell her

I'll keep the story brief, I really only want to know what my best course of action is from here.

I started dating this girl long distance, a little under two years ago. Things were good between us, but there were some underlying things that hinted at us not lasting, red flags, if you will. Every time I tried to talk to her about the things I felt were going wrong, she would get excessively hurt, sad, and angry. This would lead to arguments that would last for hours, which came down to me apologizing profusely for telling her the truth. Needless to say, it wasn't healthy.

I'm not here to make her out as being the bad guy, because what I did next was far from the best course of action. Eventually I became sick of these arguments, and tried to break up with her (trying to be as nice as possible about it). She cried and begged, and we got back together, I was too weak and gave her a second chance.

From this moment on, I didn't want to be honest with her anymore, it would just lead to arguments. So, I stopped being critical. I accepted every little thing, just let it go, sugarcoated the truth when talking to her. But she didn't change, things really only got worse in our relationship, even though I was sacrificing my own time, happiness, and maybe even sanity to keep her happy. I'd gotten over her near the start of this year, but stayed in the relationship nonetheless. For her, I thought at the time.

Eventually she had to find out, and I had to tell her I didn't love her, that I hadn't loved her for a while. I was about to start at a new college, so I had to break it off now, or regret it. This was the last week before school started. She seemed to take it relatively well, at first. However, after about a month, it became clear she was not going to let me go. She was really struggling, and I didn't know how to handle it. I didn't want to make the same mistakes I had made during our relationship, but I also didn't want to hurt her. I decided to just be honest with her, all the time. After all, that's what had ruined our relationship; the lack of honesty.

Now, after 7 weeks in my new course, I got very close with a classmate of mine. We really hit it off, and I developed feelings for her. As we'd promised, I told my ex about this, she wanted to know when and if I talked to new girls. However, this, she did NOT take well.

The last month or so has been a constant stream of me having to explain and justify simply listening to my heart and living my life, while I'm being barraged with tough situations. She's threatened to kill herself, broke down crying multiple times talking to me (in fact, we can barely hold a conversation without her breaking down now), and she's been hurling insults and accusations my way. The problem is that I "moved on too quickly", so now she believes I don't care about her, and that I betrayed her, didn't respect her feelings.

So to summarize: we broke up over three months ago, I've explained to her how much she's changed my life, but that I simply don't want to get back together with her. One day she's crying that she wants me back, the other she's crying that I betrayed her, and the next she's crying how much I hurt her and that I don't care about her or consider her feelings at all. All that's changed is that I am now honest with this girl, as to not retrace my mistakes from the past.

Everything I try leads to her getting more upset, and I can't take hearing her cry anymore.
I can't tell her that I'm sorry for moving on, because frankly, I'm just trying to live my life now.
She expects certain behaviour from me, basically expecting me to still act like her boyfriend. Being understanding, patient, and nice, while she's not any of those things to me. I'm trying to treat her like a friend, but she claims I treat her horribly. (Again, all that's changed is I am now honest about my feelings, and what I think of the way she acts)
She wants me to see what I did, and admit I am in the wrong. I see why she's hurt, but I don't believe what I did is wrong.
The straw that broke the camel's back was when I told her I don't owe her anything. She's responsible for her own happiness, and if the truth is too much for her, she shouldn't ask questions she knows will make her feel worse.
I just want to be honest with her.

Ask any further questions if necessary, I just don't know how to handle this situation or what to say to her.