Should I assume we broke up?

My boyfriend and I had a terrible argument on Saturday night. He called to apologize for being so insecure and for creating problems (jealousy, possessiveness, accusations of cheating) in our relationship, and I was just so angry with him that I kept yelling at him. We’ve had the same ongoing problems for a couple months, and we’ve never resolved them because he becomes condescending during our “discussions”. I’ve walked out on him four or five times already. Anyway, I discontinued our conversation on Saturday night, and he hasn’t called me since. I don’t know what to think. Should I assume we broke up? I don’t have the guts to call him..honestly. I miss him so much it hurts. I was thinking about just writing him a letter explaining my feelings CALMLY. I think it will provide me with closure whether he responds to me or not…at least he’ll know where I stand. That’s enough for me. Is that stupid? Or should I just let him go? I keep wondering if he’s thinking about me too…or if he’s just waiting for me to call him. We’re both very stubborn, so it’s obvious one of us has to let his or her guard down. Looks like it will be me.