Breakage can be from a variety of things.  You've mentioned several already.  You say you brush your hair.  If you hair is fragile, you might want to consider using only combs on your hair.  Brushes can be hard on the hair.

Be careful how you handle your hair when it is wet.  Combing or brushing the hair when it is wet can cause breakage.  The hair is very elastic when wet and it is easy to stretch it to the breaking point. 

As a temporary measure, so that you hair will look nice for your party, you might try using a shampoo and conditioner that contains -cones.  Pantene and Garnier Fructis are two brands that are known for their cones.  The silicone ingredients will coat the hair and make it shiney.  This coating will make the hair thicker and glue split ends temporarily. 

Some hair likes cones, some hair does not.  I usually recommend that a clarifying shampoo or treatment be used regularly (every 3rd or 4th shampoo) to remove the buildup that can occur with cones.  This prevents the hair from being smothered and end up breaking even more.

Congratulations on your graduation and good luck!