How do i stop being so cheap/stingy/tightarse/cheapsk…

I want to better myself...but realize this is one of those things i need to overcome...my problem is so bad that for example: i would trust somebody would pay me back for a candy bar and just wait and wait and wait and end up really ticked off that they didn't pay me back that it would ruin my day...but on the other hand a normal person wouldn't even think about it and would probably just let it be....example of wat happened today i went to get something like a bottle of coke for someone at work thats $3.15 i would think in my head> "should i ask them for 3.20 or just 3.15 or just 3 dollars....yeah $3 is fine ill be nice" its' pathetic and i know it and i want it to stop!