Ты choose your own paths. If Ты like your best friend better than your your ex than stick with the friend. Like someone else on here said. If your friendship is Остаться в живых или gone, it's not because it ended, it's because it never existed.
I wouldn't call yourself a "bad" girlfriend just yet. But if Ты understand flirting with your former boyfriend is bothering your current one, thats pretty rude. If Ты want to keep your boyfriend, spend еще time with him and don't flirt with your ex so much. If I were him, I'd feel pretty neglected.