You invited her to be in your bridal party.
She said no.
She's allowed to decline an invitation. An invitation is not a subpoena and people cannot be forced to accept an invitation.
She had prior plans. She's not obligated to drop everything just because you decided to get married that day.
She also TOLD you that it'd be too much of a financial burden for her. You don't get to tell her otherwise. You don't know her financial situation - and just because she owns a busienss doesn't mean she's rolling in dough.
You can feel however you want to feel about the situation, but you can't DO anything about it. You invited her and she said no - case closed. Move on with your life instead of acting like a spoiled brat about not getting what you wanted.
"she has always put herself first in the past when it came to things like this. She just doesn't seem to give a crap about weddings but I thought mine would be different."
Well, that's YOUR problem, not hers, that you believed your precious wedding would magically change her into a different person. Just because some dude shoved a ring on your finger doesn't mean that people are going to do a 180 and give you whatever you want, dear. If she was a good enough friend for you to ignore this kind of behavior before you got engaged (not "ingaged"), then I fail to see why your wedding is the magical turning point where such behavior suddenly becomes unacceptable. You're not THAT special, princess.
If it means THAT much for you to have her there on your wedding day, then either change your date or offer to pay for 100% of the costs associated with her being your bridesmaid, and see if that changes her mind.
Also ... when you say you have a "date" does that mean that you booked the church/temple/hall and officiant already, meaning you signed contracts and put down deposits? Or did you just decide that XYZDate sounds nice? If you only got engaged THIS WEEK then I'm betting you just picked a date out of thin air, correct? Your desired date might not even be available with your chosen vendors ... so it's going to be fuccckkking hilarious if you throw a bratty temper tantrum and boot this poor girl from your life because she didn't cancel her vacation to be your handmaiden, and then you don't even wind up getting your wedding date booked for the week she'll be away.