two solutions:
A (the one i favor) you be straight with her and bite the bullet. tell her straight up you're not feeling this and you want to stop, don't want to lead her on. that you don't feel her like that so she shouldn't ruin her relationship with fiance since you don't share the same feelings. she'll still be on you and still be crazy but you'll reduce the odds of her trying to hurt you cause she just might respect the honesty.
B: you con her. basically you start acting like a lame around her. start simping hard. be insecure, weak, needy. she'll be so disgusted by you she will ditch you on her own.this will mess up your name but it might save you some trouble down the line. this might not be a good idea for you though since she's at your job.
EDIT: also, save the texts, pics, emails, erytang!