Let's just state this simply and honestly.
You broke your boyfriend's heart and trust. He dumped you. The end.
You ****ed up, you're now enjoying the consequences of your actions. You had your chance to end the relationship with the other man, you failed the first time, and when you finally did close the door on that relationship it wasn't in a manner that brought closure for your boyfriend.
What's there to understand? You ****ed your boyfriend in the ass, told him you did, and eventually he decided that he really wasn't OK with it. He didn't screw you over, but he is hurt and probably doesn't really want to see you.
How is he stringing you along, if you're the one still hanging around him? He hasn't resolved his feelings about you, and he's still confused. What about it?
Sheesh!
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."