From NU’s website: If you are certain that Northwestern is where you want to enroll, we encourage you to apply under our Early Decision plan. As with all traditional Early Decision plans, you agree to withdraw all applications at other colleges and enroll at Northwestern if admitted. Students interested in the Honors Program in Medical Education (HPME) must apply under the Regular Decision plan.
Applicants who choose Early Decision send a strong, positive message to Northwestern. Given their high level of interest and overall academic and personal strength, Early Decision applicants enjoy a higher rate of admission.
If you are admitted under Early Decision and apply for financial aid, you will be notified of your aid decision around the time of your acceptance, provided your family has filed the College Scholarship Service Financial Aid Profile (CSS Profile) by December 1. Students admitted under Early Decision may be released from the commitment to enroll at Northwestern ONLY FOR DEMONSTRATED FINANCIAL HARDSHIP.
The capitalization is mine. You also potentially penalize your high school going forward to renege on an ED acceptance.
I agree wtih @RightCoaster - if you are having second thoughts OP, pull your ED application to NU and move it to RD.