I texted my boyfriend that I was going to break up with him, and he didn't return my text messages or phone calls. Why is this? What should I do? 、

If he doesn't respond, it's a breach of contract. Since you brought it up, there is no need to remind him of the existence of this matter. Why did he call and text you after the breakup? This is not a festival. You should put your mind right, whether you want to break up or get his attention.