First off, don't worry. It happens. Yes, it wasn't the best way to handle the situation. But clearly you had some unresolved feelings that came up once you discovered that he has a new girlfriend. This is the downside of the "No Contact" rule-- you thought that zero contact would make you get over him faster, when in fact it built up resentments, feelings, and unresolved emotions. Perhaps you had gotten over him, but hearing about his ex brought back all the emotions and issues from the break-up.

Send him an e-mail. Tell him that you're sorry for how you acted. It was an emotional response that came from pent-up feelings. Explain what caused you to react that way. Let him know that the news of his girlfriend caused you to verbally lash out at him. If you didn't mean those things, let him know. Apologize, wish him well, and continue to go your separate ways.

However, if there are things that you need to say to him, say them. Do whatever you need to do to get closure. If that means never seeing him after you hit "send," then do that. If you need to talk things out in person, ask if he wants to meet for coffee or talk on the phone. Do not go over to his house, invite him over, go out for drinks, or create any other situation that might lead to a fight or awkwardness with his new girlfriend. Right now you need to ensure that you won't have another blow-up. If that means talking things out one last time, propose that idea. (However, be prepared for the possibility that he may not want to see you.) The e-mail apology is probably your best bet. After that, go back to zero contact. For one thing, it's unfair to the new girlfriend if you're in the picture. You had your emotional response to his new relationship. Now it is time to apologize and move on.