I second what DubaiSis said about going back to your doctor. I tackled depression in high school and I know what it can do to a person. After trying a few different medications, I am as happy as a clam and my depression very rarely affects my life anymore. Why don't you try contacting your school counselor as well? Universities have counselors on hand that you can see for probably no cost to you. Maybe it will help to talk to someone outside your chapter. Don't be afraid or embarrassed to go see a counselor - a lot of people do it, and helping students is what they are there for!
This might be controversial advice, but I don't think that you should drop. If you drop, you're letting them win. Don't let a group of pathetic, petty girls take the Greek experience away from you. Plus, being an alumna is a life-long experience and you won't have to deal with the rude sisters in your particular chapter anymore if you do not wish to. Don't give up your sorority because of these girls. If anything, they should be the ones who miss out on being Greek because they are the ones exhibiting such negative behavior.
Another thing - if they are sending you threatening or mean text messages, Facebook messages, etc, yes delete and block them, but not before you print the messages as evidence. You can turn these over to your standards director and chapter president as evidence of the bullying.
Keep your head up and ask for help if you need to from friends, family, or your school counselor. Don't let these girls get to you. You are better than that. Don't stoop to their level or let them win. You are awesome and can get through this!
