Personally, I've always found it distasteful when an artist changes the agreed on media/size/whatnot AFTER taking payment, and NOT offering the option of a refund at the same time. Not everyone is willing to alter what they originally asked for, nor should they.

You are a saint in how long you put up with being ignored while she did other commissions in that close to 10 year period, and I don't blame you for wanting your money back and to be done with it. I seriously hope you continue to only deal with upstanding artists that won't repeat this process, gosh..

I have no advice to offer, just a lot of sympathy for you as both a commissioner and artist myself.

Edit: Grammar.