How to create a right balance between being passive and being aggressive?

I controlled my emotions and anger for years, it became a part of who I was. But I recently realized that if you don't stand up for yourself at times, if you remain overly passive, you're going to be mocked and bullied. That brought about a change in my attitude and I think I can show my anger now to someone who deserves that. I am trying to be more assertive these days. Now I am always ready to give back to the bullies a taste of their own medicine. While I feel more confident in myself now, I wonder if all that new found aggressiveness is good for me.