I understand your feeling hurt, but I think it's likely there's a reasonable explanation (rather than that she didn't want you in the wedding, or it didn't occur to her to ask you.)
When I married (at age 28), I'd had many close friendships through the years, and I would have loved to include all of my girlfriends in the wedding.
For one thing, my DH didn't feel close enough to very many men to ask their participation. He asked one friend from college to be his best man, and his boss to be an usher. I had a hard time choosing only two attendants, but finally chose my sister as MoH, and a college friend as a bridesmaid. The reason I chose her was because I was closest to her at the time, even though I'd known a few of the other girls most of my life.
Another reason we kept the wedding party small was because we didn't have a lot of money, and neither did my parents, who were helping us pay for the wedding. Aside from paying for the girls' outfits (including jewelry), which we did because neither had much money at the time, I had no idea how expensive everything wedding-related was until I was actually planning mine. (I wanted real flowers, and the cost was quite a shock!)
I hope you get over your hurt. I think you are feeling your friendship undervalued or overlooked, and suspect it was a difficult decision for your friend to choose among her friends. You may be right that, like I did, she chose whoever she was seeing the most at the time, and it is no reflection on her regard for you. I would bet that your continued friendship means everything to her.
