You can use your willpower to catch yourself every time you have negative emotion. Strengthening your will takes practice. You practice it every time you reject instant gratification in favor of a higher and better goal. One simple daily practice to develop will power is by refusing to satisfy trivial desires (like channel surfing, snacking or drinking soda). By learning to refuse to satisfy every desire that crosses your mind, your will gets stronger. Practice it often during the day, delay yourself by 30 minutes getting that soda or turning on the tv. That’s practice. You’re exercising that muscle, like going to the gym, so the strength is there when you need it.