Get your head in order and learn to be responsible. Sorry, there is no magic way to stop yourself from buying other than being responsible or like what you are doing, just don't buy it. Of course easier said than done, but be glad that you are only psychologically addicted marijuana, and not physically or psychologically dependent on another substance, as marijuana is probably the easiest as it gets in terms of addiction.
I'm in a similar boat myself. I've been smoking like a fiend and have stop finding motivation for some of my major responsibilities. The fact that you, and I can recognized that there is an issue with our use (not the drug), we can take the steps necessary to move forward.
What I recommend is taking a break (I'm on day 19 atm), and let the fog out of your head (figurative of course). It'll take a few days to stop being irritable, sleep a restful sleep, eat a full meal in a sitting, etc (of course this depends on how the psychological dependency carries over to some sort of withdrawal for you, you may not get these symptoms). However, after a week or so, you should be able to function through your days with out much issue. To potentially get your head out of the clouds (the feeling of forgetfulness, trouble doing fast calculations in your head, etc.), it may take a few weeks longer (though it varies from person to person, if this is a problem). At this point (3-4 weeks) your tolerance should be practically non-existent, and if you feel like you can start using a limited amount, buck up and get that self control rolling. However if you don't change your daily habits (doing other stuff besides smoking weed/avoiding activities you do while your high) in the time that you are taking a break, you may fall right back into your usual patterns of use. So, I recommend trying to set up some good outlets of energy, creativity, etc, to start to create new healthier patterns in your life. Once you feel like you can incorporate marijuana back into your life in a occasional, healthy way, go for it, but make sure you don't fall back into your old ways.
Now I need to listen to my own advice
