Mighty, I'm so sorry you are having a time of it. The best thing to do is to leave him to his mess. You can't rescue him and you can talk to him about the relationship and how you feel until the cows come home. He's saying the right words, but his actions are saying something else. He can't have his cake and eat it too.
If you told him not to step into your house, then you will need to stick to that boundary. Whatever boundaries you make, stick to them and no changing them up just because you become a softie because of something he tells you.
He's torn between two lives. He's not going to make a decision about which one because he confused and stressed. He doesn't know which life he wants, but he's got to make a decision and the best way for him to do that is one of you needs to step outside the boxing ring and let him figure it out. Mighty, step outside the ring and give him all of the time and space he needs to figure things out.
Don't be angry w/yourself or him. Both of you are in a whirlwind and I have seen this happen to others. They say they want to reconcile and then it becomes a waiting game for this or that. Why? Because if Plan A (ow) doesn't work out, they know that Plan B (you) will be there for them. I'm not saying this is what is going on w/your xh, but many of them do this. If he truly wanted to reconcile and actually work on the relationship w/you, he would be doing it. Sure, he could visit the baby and accept responsibility for the child, but that doesn't mean spending the night over there or jumping when she says how high. Your man is still a teenager and hasn't quite figured out that he needs to make some mature decisions.
As for the cold sore, he's looking for sympathy and wants you to say something to make him feel better. I wouldn't acknowledge the cold sore. He's a big boy and the cold sore will go away when it's ready. You do realize that it may have come about due to stress? He's trying to get your attention and the only way to do it is to tell you he's not going to D13's game and that is the silliest thing to stay way because of a cold sore. Poor thing, vanity runs deep.
Any way, please take care of yourself. You will be okay once you get your balance back.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.