What Guys Said 2
I know it's hard, but I have to say this. You desperately need closure. He chose a girl over you, in impulse. No hard decision making, he just made it. This means his feelings for you, while genuine, weren't enough to stop him from going back to her. Get over him and move on. I'm sorry.
What Girls Said 2
All you can really do is wait if you really want him back. He could well make a similar snap-decision to dump her and go back to you. BUT if he's that willing to give up on your 5 months so quickly for someone he dated for a month is it really worth having him back if he valued your relationship so little? Your choice, but in my opinion you'd be doing yourself a favour by getting out of there.
I wouldn't take him back even if he did come back. It sounds like you're second choice right now. He may or may not regret it depending on how things go with her, but I suggest you move on. He'll only start to miss you if things don't work out with her. You should not be okay with being second choice or backup.