I would say for myself, 98% of the time I can be pretty good... it is that 2% of the time that I can lose it.
A friend of mine had a family member who quit drugs and drinking and stayed clean and sober for 27 years. He then took a drink and was dead in less than a year.
That's why I love that quote by Pa(b)st Prime about being disciplined on EVERY trade. You never know if one emotional trade will cause a little damage or start a "binge" of emotional, irrational trading that wipes you out.