my budget is killing me

I put together a budget and decided I was going to stick to it for the month of July.  It seemed comfortable.  I was going to try it for this month and tweek as needed.  Not even a week after I create it I forgot to add my car registration and inspection stuff.  $285 dollars to reregister and inspect my car.  Does anyone have a budget that works for them?  I want to save $500 a month but it's starting to look impossible....help.