How do I stop seeing a guy who can't give me his undivided attention?

No, first off this guy cant be trusted, if he is doing it with his ex he will and is doing it to you. Sounds like he is just a player who wants many girls, and says what he needs to get them. This will toy with your emotions and may lead to a lot of hurt in the future. Please, you deserve someone who will be faithful and you can trust.... Most people like this wont stop even though they say they will