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The reason is probably you and your habits. If you are salaried person, start saving at the beginning of the month. I will tell you what I personally do.
I receive my salary on 1st working day of every month. By 5th, all of my investments are done from my bank account. I have setup my banking account with debit mandates which automatically transfers my money into predefined investment plans and wealth management accounts. So by the end of 5th, you come to know your financial power and then start spending accordingly.
This method really helped me. You may want to try.
Wow! you seem to be the kind of "man with a plan"
I personally don't think I need to do any of that, but it definitely seems a great system to me, for those who have difficulties managing their cash flow or want to simply have more peace of mind, financially speaking. The OP seems to be that kind of person, so I hope your post doesn't go unnoticed.
Little tip: keep using cash and avoid cards as much as you can, as a simple habit in your daily life. Better to educate yourself to reduce your spending habits rather than to give away the data of your consumer behavior to who knows who because of such a childish excuse (because you can't resist the temptation otherwise? really?).