Dont beat yourself up too much - at least you had protein instead of carbs. It could have been so much worse. I doubt that you would have been in Ketosis that early so you may not be back to square one just for a little bit of milk. I am sure that you are ok and you will be able to do this. Take it one day at a time and before you know it your first week will have passed and you would have lost loads
Its only weak of you give in. Being strong is getting straight back in there and never giving up! It takes some getting used to honey. Once you get into ketosis it will get so much easier.