It's one thing to sit down and write down your budget. It is quite another one to actually stick to the budget. Your first month is usually the toughest when it comes to sticking to your budget. You may have underestimated some costs, and overestimated others. When this happens it is okay to move money from one category to another.
However, actually decreasing discretionary might be significantly more challenging. Everyone has different weaknesses when it comes to spending money. Some people may have a video game problem, other people a shoe problem. It is best to allow yourself some money to spend each month, otherwise you may entirely blow your budget. It should be enough that you can buy one reasonably priced item in your weakest category. You may want to use the envelope system for this category. What's your weakness when it comes to discretionary spending? And how have you learned to control it?