Hmmm maybe you should go see a naturopath, there could be an underlying reason for your body wanting sugar so badly and reacting the way it does when you don't have it.
I used to need to have chocolate every day and if you can survive two weeks of the cravings without having it you lose the cravings. Don't buy the food, give your key card to someone if it means you won't buy it. Do a weekly shop and stock the cupboards with healthy foods. Diet shakes pretty much set you up for disaster I think as your not satisfying yourself at all.
If you really know you just don't have the will power to do it then get yourself into a group with ppl who are struggling with the same things so that you have the support.
Could there be reasons why you are eating unhealthily? I mean emotional?
Even if you cut out the sugary drinks that would make a massive difference, at one point I was addicted to drinking coke zero, I think it was the sensation you get when you drink it, it's so fizzy.
I cut it out and switched to cordial and now can't drink it.
Wish I could help hun, all I can say is I believe you can do it I think you just need someone else who can help you, hold you accountable.
Good luck