Mia,
I sold several of my Chanel handbags. I had known for a long time that eventually I wanted to start purchasing Hermes exclusively. One day I looked at all the Chanel handbags I had, some never carried tags still attached and thought OK I'm ready for Hermes.
I still have 6 Chanel handbags I need to sale and I am contemplating selling my Chanel costume jewelry and my Chanel J12 Watch. Chanel is a great brand and has beautiful items, but I REALLY only want to own Hermes. I have "fallen in love" with the brand for its quality, history, excellence and overall appeal.
Maybe selling some of your designer items might be a way to establish your savings for your Birkin. I understand your feelings - once I made up my mind, I would not be deterred.