If there was another girl...
If there was another girl, there will always be another girl. Even if he stops talking to that girl, there's always a chance of them talking to each other again. Even if they do completely stop talking, there's always a chance of there being a different girl coming in to the picture. If he left you for another girl once and you take him back, there's always a chance of him leaving you for another girl again. You'll never be the only girl for him because you were never enough for him, and that's something you need to look past your heart and realize. He will never change because he doesn't want to change, and that's something you need to look past your feelings and accept. He will always be someone who will lie to you because he doesn't care enough about you to be honest, and that's something you need to look past your emotions and understand. He's already proven that he can be easily distracted, he's already shown that he can be easily manipulated, and he's already demonstrated that he can be easily tempted. As much as you'd want him to be, he's not the right guy for you. Stop forgiving what shouldn't be forgiven, stop believing that he deserves you, and stop trying to force something to work that's not meant to. Please don't go back to him, because you can't find happiness where you've lost it.