It sounds like you don't have a lot of options. You are obligated to the LL and to your roommate. That your LL is willing to sublease at all is nice, many won't. A sublet won't get you off the hook, though. The person you sublet to will pay you and you're lease is still in effect. If you sublessor doesn't pay you still have to. At least this is my understanding of a sublet.
You probably won't be able to get out of situation with a sublet, however. Your roommate has made her feeling clear, she won't have another roommate next year. You cannot compel her to live with someone else simply because you changed your mind about school. This is not your roommate's problem. IT's totally legal for her to insist on you honoring your obligation.
You do have one out, though, but it's not pretty: move out. Move out and see who flinches. OF course, in doing so you shoulder a rather substantial amount of risk. The landlord can sue you and your roommate can sue you and, as you have a contract, you are likely to lose and take a big hit the the wallet and in the credit history.

