I know how you feel 100% We have gone through a lot with our DW plans as well. All of our friends had at least a year and a half of planning notice for our DW. So it wasn't a surprise. I thought I was being smart asking my wedding party well in advance so I knew who could afford it and who couldn't.
My best friend since junior high backed out on the deposit deadline as well. But by this time we were in full wedding plans. I had already put a hair and makeup deposit down for her and getting all the girls set up for their bridesmaid dresses. She was involved in everything!!! Then I get an email (to my work account) that her motherly duties are too important and she could make it. I was so upset and I'm not one to cry at work but it hurt too much. First she should of called and told me as a friend should? Second why not say that from the very beginning?? If your motherly duties are so important weren't they important 8 months ago too when I first asked you to be apart of my wedding day? So not only was I out a bridesmaid but I was out my deposits I had put down on her.
Like yourself I have many friends who married at a early age and now have a family. I've attending all of their special events, wedding, housewarming, baby showers, birthdays etc. Hell, I've hosted a lot of these things for her and said a speech at her wedding. All I wanted was her to be up front and honest. I sent her an email back telling her how I felt. To this day I have never heard back from her. It still bothers me. I thought she was my best friend but now I just see I was HER best friend.
Bless my fiancé heart he as been so supportive though all of our wedding drama. If you knew me I hate drama in my life and I thought a DW would be easier then a wedding back home. It really isn't . It's just different drama you have to deal with.
I agree with all the girls on here; having a wedding you'll find out who your true friends are. But the people to focus on are the ones who are sharing your special day with you; and to have a true appreciation for the people who wanted to be there but couldn't and were honest about it from the beginning.
Best of luck to you. You aren't alone.