I effed up, Knotties. Lurkers - this is why you don't ask friends to be bridesmaids too early

The wedding isn't for another year. I got caught up in the excitement of getting engaged, and asked my friends to be bridesmaids WAY too early (and way before I started lurking on these boards). For the most part, this has been an okay decision. I chose 4 friends to be bridesmaids and 3 of them I have been friends with for 8+ years. These are steady, long time friends who have survived sporadic and long periods of not talking to each other without any negative impact. Even with a year away, I have confidence these friendships will stand the test of time!
However... I asked a relatively recent friend to be a bridesmaid as well. We'd been work friends for about a year and half and became very close, very quickly. It made perfect sense to ask her to be a bridesmaid, given how close we were. At the time I considered it almost insulting to NOT to ask her, because of how close we were. I couldn't possibly see us not being friends.
There's been a lot of changes since then, however. She has changed tremendously as a person, and part of me wants to justify my poor decision to ask her with the fact she simply isn't the same person I asked. While there are many reasons our friendship has died out, I don't feel the details of why it died are necessary to air out on here. The gist of it is - we began growing apart, she broke my trust (nothing wedding related), we tried to rebuild said friendship, it didn't work. To avoid uncomfortableness at work, we agreed to just start over and work on rebuilding our friendship instead of trying to repair it to what it once was.
Sounds promising, right? But it's not. I really have only done this as to not impact our work place. I am moving to be with Fiance while he finishes school. I am 100% certain this friendship will dwindle out after the move. What I need help with is a tactful and straight forward way of stopping any awkwardness when FI and I fly back into the states next year to get married. I am almost certain she doesn't expect to be in the bridal party, or even invited to the wedding at this point. I'm trying to avoid being a total bitch by not saying anything to her at all, and politely get rid of any confusion for her.
Let me be perfectly clear here: our friendship dying had NOTHING to do with the wedding, her being a bridesmaid or anything related. I'm also well aware this decision is a "friendship ending move." Since there is not much of a friendship there anymore, this is not really a problem. I'm not looking for cristicism on our friendship - what's done is done. We both made mistakes, and it's in the past now.
My plan is to say something along the lines of "We were very close when I asked you to be a bridesmaid. I'm sorry to say we have drastically parted ways since then, and I wanted to relieve you of the burden of being a bridesmaid." I won't rescind her invitation, however I sincerely doubt she'll want to go.
Thoughts?
Sorry for the long post, I feel the need to yammer on incessantly when I dun f*ed up

EDIT: I've got a lot of great criticism on this, and have decided to say something more like "We were very close when I asked you to be a bridesmaid. Our friendship just isn't what it used to be, and I don't think it makes sense to have you be a bridesmaid anymore."
This shouldn't come as any great shock to her, so the direct approach is probably best.