Wedding Dress Troubles

So.... My wedding dress is going to give me trouble.

A while ago, we went to David's Bridal and they were quite snippy and rude so we left without setting up an appointment to try on some dresses. I was convinced that I didn't want to do the whole trying on and modeling thing - it wasn't "me." Instead, my mother, grandmother and I were going to make the dress. We couldn't find a pattern that suited me. We left it at that and I searched around on the internet. Now, I'm pretty sure that I would like to buy a dress from a bridal store, but my grandma still thinks that we're going to unconventional route and making the dress.

She called me a few days ago and had me go online to look at this pattern. The dress in the lower, right-hand corner (blue) is what she wanted to show me. It's pretty, right? Absolutely. I'm worred, though, about it looking costume-y. I told her that we'd wait until the patterns went on sale somewhere to get it, but she "just couldn't wait" and went out to get it the next day.

How do I tell her that this isn't what I'm going for without hurting her feelings?

I have no idea.