Sometimes i think we may bottle these things up when maybe the customer is just not aware of the problem they are causing.
Face to face would be best and just say that due to her not having the backs done very often its taking you twice as long so unfortunately you either need to do them everytime or charge a premium, you also could just do the front only if she prefers. Just be honest and ask her, teel her if none of that suits her then you are afraid you wont be able to clean her windows anymore as you are really busy and its taking too much time. She will either understand or she will dump you, but by the sound of it she needs to be dumped anyway so why not take a chance and ask the question.
I know, its easier said than done, most of us dont like confrontation and just dump, thats what i would probably do lol but i just think sometimes we might win a few of these situations if we simply communicate with confidence and self respect. If you allow them to walk all over you they will, there needs to be a mutual respect in this job.