Exactly. If you don't respond, that looks even worse. As a matter of fact, you have waited too long! All women secretly think they are the sh!t and will think of the best case scenario when you don't tell them what's up. Often at your expense! (This can work in the opposite way too, but only if she was into you). In this case, she thinks you are still in love with her, and always will be forever and ever more.
Call her! ASAP! Tell her you haven't had time to respond, yadayadayada, and sorry for the misconception, but you meant to dial Exandra, not Ex-girlfriend. OK!!!