How did his feelings changed so quickly?

So I was dating him for 8 months. And all off sudden he just broke up with me, because he couldn’t do it anymore. Yes we had arguments, but I never tought that it was so bad. So we’re still talking, it’s been about 4 day’s since we broke up. Before we broke up, he said that he was crazy about me and stuff. That day I was reading an old message from him and I was ready to write him a long paragraph why are we meant to be and stuff, but he just suddenly broke up with me. He said that he still has feelings for me, but doesn’t want to fight for me anymore. So of course I was beginning for him to change his mind, and etc. But he just didn’t wanna give me the chance. Do today we were talking, and I said that I feel fine and can smile for the first time and he said, that he won’t be fine for some time. And the he kinda stoped replaying to me, does that mean that he still cares? I really want him back, I don’t know what to do. His girl bestie said that, he isn’t replaying to me, because I said that it’s easier for me, but then again he was replaying before, that I told him, that I was fine with our break up. so is he still into me? And what should I do to get him back?
How did his feelings changed so quickly?
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