You know when you have a best friend but then you realize that they're YOUR best friend but you're not THEIR best friend? That happened to me a few years ago. This wasn't just any friend. To me, she was a she-would-be-a-bridesmaid-in-my-wedding kind of friend. After hearing the news of her engagement, I was eagerly awaiting my bridesmaid's duties. Days turned into weeks and I still hadn't heard anything. After a few months and no phone call, I realized that the bride hadn't lost my number... translation: I wasn't a bridesmaid.
The wedding was a whole new kind of uncomfortable. Throughout the ceremony, friends of the bride would get up to read a poem, sing a prayer and say something sweet about the couple. I'm telling you, knives through the heart. The real kicker was when I overheard someone say, "I'm so happy I was able to find a yellow dress! When the bride called and told me that special people were going to be wearing yellow..." My ears couldn't take in anything else. My heart was already broken.
It seems obvious, right? "If I don't ask her to be a bridesmaid then she isn't going to be one." You know your relationships with your girlfriends, so you know who is expecting to be in your wedding. There are numerous reasons for not having someone be a bridesmaid. You might want to only have your sisters if you have a lot of them. You may have other closer friends who you would rather have in the wedding than her. She might just be annoying and you don't want to deal with her closely for the next year. Any of these reasons are completely understandable but if this friendship means anything to you, you'll go ahead and approach this situation head-on.
First, I suggest finding a way to incorporate her into your big day so that she knows she is still important to you. Reading a poem during the ceremony is an excellent way to involve your girlfriend. Another idea that I took from my non-bridesmaid experience was to ask her to wear a dress that matches the colors of your wedding. I swear, if my friend had asked me to wear a yellow dress, it would have made the sting of not being in her wedding less painful.
Second, after you've decided on what role you would like for her to play during your wedding, I suggest inviting your girlfriend out for coffee to explain to her that she isn't going to be a bridesmaid but you would like her to do A, B, C or D. Nine times out of 10, she'll be happy and relieved that she wasn't completely left out.
I know this seems like a lot of work but try it. If she isn't happy with the effort that you made, there isn't anything you can do at that point. At least you'll be able to say that you thought of her and incorporated her. I promise, you'll thank me later.