You know, it would be one thing if it was 1987 and everyone was having mixers every week and you were going to them all. But it isn't, and you're not.
Your decision was not because you didn't want to do the work or help the sorority - it was for reasons that most 50 year olds have never had to deal with, let alone college students. And I KNOW you have not been the only person that went alum or inactive because of $$$. Maybe the dues structure needs to be rethought...I don't know, I probably should not go there since it's none of my business. What if you had a sister who joined her senior year, graduated, stayed in town and kept coming to things? What would be the difference? Like Tracy said, you joined for life.
At any rate, your reasons for going alum were obviously good enough for nationals...unless they come down and say that you can't do things that you are doing, keep hanging in there.
