Perhaps try to make the decision who to be with independently of who is waiting in the wings? Do you want to be with C or not? Make that decision without considering that E wants to be with you. Choose on an absolute scale rather than a relative one. If you decide not to see C anymore, then move on to the next decision: get back with E again or not? If you are looking for a long term relationship, then it sounds like E is not the one--you just don't feel that serious about her. If you are not looking for a long term relationship, be frank with E. Tell her you enjoy your company but it will not go further. Then see if she still wants to be with you, given the limitation of your feelings for her. Be prepared that she may drop you.

Good luck.