Definitely do NOT go back this time. You are probably just feeling a little burned that he is with someone new and is kinda serious about it. And when he's nice to you its probably because you guys were together for so long, its like routine, he's used to it. But seriously you can do better, keep your eyes open to meeting someone new and eventually you will forget about it. I know its hard but look how he treats his new girl, and keeps you on the side...that means if you get back with him, he'll have another girl on the side. Let her deal with his drama!!!