Did the bride ask you, way before now, if you'd tried on your dress that you say you've had for THREE MONTHS? If so, what did you tell her? Why did you get the dress so far in advance? Is it wrinkled? Usually these dresses are pressed and packaged so as to be ready for the big event but if you've had it for three months, it may need some touching up to look presentable.
Have you talked to the other members of the wedding party about their dresses? If not, you should as your job as maid-of-honor is to assist the bride and she undoubtedly has a lot on her mind.
Have you purchased your shoes yet along with anything else you might need for that day?
I do agree there is a big difference in something that won't zip and something that can be with help but it sounds like this dress is just too small and zipping it with help may risk breaking the zipper in addition to being extremely uncomfortable. Is it tight around the armholes if it has any? I am very curious now about this dress so would love a picture or a description such as style, size, color, etc.