Best friend issues

So my best friend and I have been friends since 2010. We really didn't hangout together until 2011 when I went through a break up with my abusive ex and she happened to move back to town. Over the years we've had our differences, personal ups and downs, but we're always there for each other... until about 10 months ago. Since then she's been more bitchy, selfish, over bearing and just flat out negative when we hangout. It varies from time to time but overall its been getting worse. I recently got engaged and of course we've talked about her being my maid of honor which in the past she's expressed interest, but now she seems to be flaking on the idea. I took her on a trip to one of her favorite places for her birthday a month ago and we had a great time, but things went right back to minimal talking. I asked her if she could come with me wedding dress shopping after the trip to which she replied she'd be able to and put it in her phone. Well yesterday was the day I had my appointment and when I reminded her she said she already had plans! I'm more of a passive person and try to avoid confrontation, but her behavior makes me beyond upset. I feel if I bring it up she'll get angry or be a super bitch about things which would crush me as we've been through hell and back with each other. Sigh... I know she's becoming a toxic friend, but I don't want to give up on our friendship. Abby one had a friend like this or an idea of how to approach het? I scheduled bridesmaid dress appointments for next month so I'm worried she'll flake on those as well.