Hey don't be so hard on yourself, you are not useless just cos you can't give it up.
I think I said in a previous thread, I kept one for the best part of a year and it took me a while to wean myself off it (still count syns). Even now I write it down if I am having a "strict" time or want to be good.
A food diary has many benefits - It makes you think about what you are eating, you can see how many speed/superspeed/fruit & veg you are having in a day, at a glance and also if you have a great week then you can refer to it again!
If you have had a bad week it is something you can show your consultant too as she may see something you don't.
So, you are not useless. Keep the diary, it's not a problem!