Hurt my sister's feelings

There will be lots of little things later on that you can involve her in (bachelorette party/shower), but for now if you want her involved right away, maybe have her help pick accessories to go with your dress, like your jewelry etc.?

I'd call her up or take her to lunch and tell her exactly what you told us. Let her know you wanted a special day with your mom and had no idea you'd find your dress so fast. invite your sis to your first fitting, and/or bring her with you to shop for wedding shoes or veils or accessories. It's going to be ok, you'll find lots of other ways to include her in this process

Stop feeling guilty. You had the experience you wanted for your dress shopping and there is nothing wrong with that at all. It is a shame she feels sad but that is her making this about her.

Take her flower shopping or have her over for a night of figuring out decorations if she is so keen to help. It is wonderful that she wants to be involved but she needs to realize she needs to be invited to be involved and not be all butt hurt when something didn't go her way. She can't go into this with preconceived notions of being involved in everything.

Don't beat yourself up about it! I agree with PP that you could definitely involve her in shopping for accessories, coming up with ideas for decor, etc. One of my sisters couldn't come dress shopping with me because she was away at school but I had her come with FH and me to sample food and cake for the wedding, maybe you could do something like that so she feels involved?