How can I break this habit?

I have a terrible habit that I need to break. I start each day great - I work out, eat super healthy, have all these great plans, then at about 3pm I basically just give up. I start eating junk food, drinking beer and basically give myself permission to "start over tomorrow". I gorge on a bunch of junk and ruin everything. I can easily eat an extra 3000 calories in an afternoon/evening. This habit has become so ingrained that I don't even think about it half the time...it just happens.. I know this is stupid because it's my own doing but it is actually a really big obstacle for me. This happens almost every day! I don't understand why I keep doing this. Can you offer any advice? Thanks :-)