How do I fix this? Now she knows what I really think of her?

Sounds very similar to my situation with my sister. All I can say is that I am PROUD of what I have worked for. I look around me and know that I built what I have. Where is the pride in mooching off everyone else in your life like your cousin does? It can feel like swimming against the tide sometimes to be so responsible and have someone else handed everything while being irresponsible, but one day, it will all catch up to her. Instead of resenting her for the life that was handed to her, focus on being proud of the life that you have built for yourself. Apologize for the outburst and say it was PMS and stress or something just to keep the peace. Easier said than done, but just know other people have been there too!