It's a "behavioral" issue, not a "logical" one.
You're not unable to comprehend reason, but obviously your issue is with your natural tendencies/impulses. Thus, your solution needs to focus on how to help train yourself to resist temptation/impulse purchases.
I've heard one thing to try is to go on a "cash only" diet. Leave your credit card in your wallet and don't touch it. Spend only with cash what you allow yourself budgeted for that week/month. If you find yourself running out, then hopefully you can pinch yourself or something.
Negative reinforcement. It works. Kidding. :)
If your weakness is fast food, then try rewarding yourself with a "once a week fast food treat". Going cold turkey works only for those who are very strong willed. If fast food can overpower you, then you are fairly weak. Not a judgment, just a fact. So, try something different. Give yourself positive reinforcement as well. Set goals, and rewards for attaining those goals. It's like kindergarten all over again...
Good luck!