Yes I sure know how that goes!
Sometimes I am just not in the mood even though I "want" to fast. And then sometimes it all falls into place and 8 days is a breeze.
Maybe its not time yet. But if it is, and you know your weaknesses (...evening eating) then set yourself up for success. Plan ahead, have something else to do in the evening when you think you might be headed for the kitchen.
Or just go to bed! Sometimes a nap or full nights rest really works wonders for me if I am wanting to cave in to temptations.
It's just those first few days that the mental chatter has such power - keep your eye on the prize and when those feelings start to overwhelm - tell yourself this is just part of the journey, feel it and direct your thoughts elsewhere. Soon you will be fasting and full of calm.
Hang in there and be gentle with yourself.
Best wishes on your fast.