HELP unwanted bridesmaid :(

HELP! I'm planning to get married later in the year, I want it to be a really small wedding with close family and have chosen my two sisters and my best friend to be bridesmaid
When we announced our engagement back in October, one of my old school friends (who I hadn't spoken to for 12 years but have re connected with about a year ago) asked immediately if she could be a bridesmaid, I replied that I hadn't even thought about that sort of thing yet.

Theres no solid reason I can't have her - I just want to keep things small, and I feel that if I were to have four bridesmaids, there would be others I would consider before her.

I took her out for a drink a few weeks after and tried to break it to her, but she was practically begging me, offering to buy her own dress and going on about how she's always wanted to be one, she even text her mom a pic of my engagement announcement in the paper so i'm worried she's told her too!

I didn't say she could be, just said i'd see. Haven't spoken to her at all since then so was hoping she'd got the hint - i feel like i've been denied my choice, i really enjoyed telling the girls i have chosen and watching their reactions, calling them about dresses etc... Anyway (sorry to go on, just want you to get full picture).

I just put on facebook that i've been looking at bridesmaid dresses, and she's replied 'nothing too horrific please' i've deleted that to save her embarassment but WHAT DO I DO!

Has this gone too far? Should I fold and fit her in - but everyone tells me it's my wedding and I need to do what I want. I don't want to hurt her feelings, but she hasn't even been in my life for 12 years!

So sorry to go on, but i'm at my wits end - I feel like I could cry!