You know what, she sounds like a deeply narcissistic person, who thinks the world revolves around her, can guilt-trip you into including her in the first place, and places personal gratfication over and above your feelings.
Your wedding day is YOUR (and future DH's of course) day, not another occasion for her to make all about her needs. If you did change it, she'd no doubt come up with another reason to cancel the new date. She is trying to make you jump to her tune to stroke her own ego - a classic narcissistic trait, IMHO.
I realised my mother was a narcissist on my wedding day. I had naively thought surely this would be the one day of my life when she would allow my wishes and choices to be respected, where she might pay me a compliment? No such luck. She disregarded all my choices and changed venue, menu, invitations behind my back. She chose my outfit and told me to have my haircut in advance as the stylist would be too busy on the day (doing mum's hair of course!). She made her own grand entrance as mother of the bride at the reception, whilst I was given her bag to carry. I was stupid to allow myself to be treated that way but realise now it was a lifetime of conditioning to be doormat/scapegoat.
There's some good threads in relationships on emotional abuse by mothers and narcissism. I suggest you have a look around there and see if anything sounds familiar.
The fact that you are even questioning this to me suggests you are conditioned to put her needs before your own or you are subconsciously seeking her approval. There is much to be learned on these boards which will help you understand these relationship dynamics.
In the meantime, enjoy your wedding and remember it's your big day, not hers!