Practice mindfulness. You don't have to actually meditate, but just try and take some time each day to conciously think about something that made you feel emotional, and why you reacted the way you did; I normally do it while taking a shower. Eventually it'll become a habit where something happens that would normally trigger an emotional response, and you'll be able to react in a way that helps with the situation instead of just having an emotional outburst. You don't have to stop feeling emotional, but you shouldn't act purely on emotion.