He's not a bad guy, he's a very confused guy. It's obvious he is still hung up on you so bad even tho he tried to move on with this other girl, it wasn't enough to stop him from seeing you. This girl he is seeing is just a rebound, you are the one he wants to be with. The trouble is you have been so indecisive, insecure, up and down with him, it has him pretty messed up. I bet his friends have been telling him to move on, go out date girls but he can't help himself. He's not a cheating lying bastard, being some kind of player, not at all. People in love get desperate, don't make the best decisions....He's desperate, you make him desperate, desperate for things to be as they were, and keeps living on that hope. Hope is why he won't tell you about this girl. He doesn't want to mess anything up if there is any hope getting back together. Remember this is the state of mind he is in.
So I have to ask you, Xox-Zip-xoX I, What do you truly want? What do you want with him? What do you want in life? Where do you see yourself 1, 2 3 years from now? Do you really have any idea or direction? You need to get your s hit together on this. Make up your bloody mind, and communicate with him, and stop playing games. If you know about this other girl, just confront him about it, but do it with understanding that he is confused. if you talk it out, you can work it out.