You did not undo all your hard work. What is done is done. However, you may want to take time to journal about what happened yesterday to send you to the store in search of not so healthy stuff...Were you bored, lonely, angry, tired or feeling as though you have deprived yourself for so long without a bite here and there, you caved?
Remember you can choose to make healthy choices at any time. You do no have to wait until tomorrow, the first of the week, the first of the month, etc...you can start right this minute.
Hang in there!