fall rush?

Why are you regretting it? The whole process is very unair and it's run by people who are barely adults. It's too much about popularity and not about ethics and ethical behavior. Check out the procedures or lack there of for getting in. It's morally unacceptable and detrimental to one's health. There is too much emphasis on drinking and not enough on social responsibility. MU is a great campus with so many more opportunities for you.

some of the chapters do fall informal rush for upperclassmen. not all of the chapters do it, but several did this fall (I think pi phi, aopi, fuzzie, and agd). it depends though, just keep an eye out next year. it doesn't necessarily mean you'll get a spot though cause with formal rush you're almost guaranteed a bid while with informal they might have a certain number of spots they're willing to take. goodluck!

i'm a boy, but thanks, i think it might be the same for fraternities

Some fraternities will even have a second spring class if there are enough men interested. The rush process is much less formal for other classes besides the main spring one. You will have to work whatever connections you have with people you know in fraternities or give your number to frat guys you meet at parties, maybe contact the chapter president or recruitment chair as listed on the university website or wherever that info is, you might even be able to get your name on the rush list so that you can get emails from chapters holding recruitment event. You will have to do more work on your own but you will be able to get into a fraternity if you put in a little effort to get your name out there. Its common for people to be in your position after they see how much their friends enjoy fraternity life. Good Luck!