If he can cheat on you, he will do it to the next girl that comes around. And you deserve so much more than that. If it was easy for him to start talking to another girl and move forward with another relationship while you two were still committed, he isn't the right one for you. It may be hard to hear and I know it is hard to go through, but if he isn't able to give you his full heart he's never going to... and you deserve so much more than that.
If he can cheat on you, he doesn't truly care about your feelings. The heart breaking feeling when you find out is one of the worst feelings in the world. As much as it hurts, it is for the better. It's a hard question to ask yourself but, wouldn't you rather learn about his cheating ways now instead of when your relationship gets even more serious?
If he can cheat on you, he doesn't know or respect what a committed relationship is. A committed relationship is between two people with open communication and an expanding amount of love for each other. There shouldn't be a need or desire to break the trust you have in your relationship.
If he can cheat on you, you should now that you deserve better. This is the hardest part. You have to get through the anger, embarrassment and confusion. You'll come to the conclusion and realization that there is someone out there that will treat you right, have your back 100% of the time and care for you like you deserve. It's not going to be an easy. But from someone that has been cheated on, I promise it will be ok.
If he cheated on you, it will make you a stronger person.