Look, you have asked our advice but we can only give advice on what you tell us.
I’m giving you the other point of view because it’s a bit sad if you lose a long-standing friend who you are very fond of because you have taken the advice of people on Weddingbee who do not know either of you. I have no particular point of view to defend and I do feel sympathy for you.
My thoughts on this matter are as follows:
It would have been better if she had been honest about you not being a bridesmaid rather than leaving you to find out on Facebook. That is certainly something to discuss with her. For the rest just be clear about what you want.
Do you still want to be friends? If so, then you are going to have to be pleasant and celebratory. Do you want to punish her? Then get ready for a shouting match and maybe no friendship at the end of it. Do you want a better friendship at the end of the discussion? Then be firm and honest but kind and thoughtful.
If you feel that she is using you and if this is definitely part of a bigger pattern of behaviour then you have some cause for grievance. You need to talk to her about not only the hurt from the choice of bridesmaids but the other hurts you have experienced. Then you need to be clear that being used is unacceptable.
If you feel that she should have been honest and straightforward, rather than avoiding the embarrassment of telling you that she already chosen bridesmaids and had not chosen you, then you must ask her why she didn’t tell you this straight away. Once you have done this then get on and help with her wedding but be clear that you would rather have honesty plus confrontation rather than a lack of honesty and a lack of confrontation. [Avoiding confrontation and embarrassment is a bit cowardly but often goes back to childhood where children are actively discouraged from facing such things. It’s a bad defence mechanism and one she needs to discard.)
If you are simply hurt that she hasn’t reciprocated in a like manner and you view this reciprocation as a measure of your friendship then you must talk to her about how you feel and also how she feels. You can’t be her bridesmaid now because you will never know whether you have forced her into that decision. She may herself have been forced to choose other people through family pressures. It may be that she has lots of friends and couldn’t choose them all. It may be that you value her more than she values you. You won’t know until you talk.
If you feel that you are happy to do the make-up and music but need her to share the costs or pay for it all or recompense you for lost earnings then you must say this to her in a straightforward manner as a business transaction without too much emotion attached.
If you are not happy to do the make-up and music (perhaps you have work to do right up to the wedding or perhaps you would find it difficult to have to put makeup on the bridesmaids) then let her know as soon as possible so that she can make other arrangements.
If you just want to punish her then I don’t really have any advice to give except to warn that punishments often hurt the punisher as much as the punished.
If in your heart of hearts you know that she is a fair weather friend who is no help in times of trouble then be pleasant, go to the wedding if you wish, but start to emotionally disentangle yourself. Perhaps it’s for the best that she hasn’t chosen you to be her bridesmaid. It makes the disentangling a little easier. Then actively look for new, truer friends. (This situation happens sometimes and it is no fault of yours. You have been through rough times and you deserve to be treated well.)
I do hope that you manage to sort things out amicably to everyone’s satisfaction.
And just in case any of you all are wondering after previous comments, I do not have a tendency towards saying anything silly. 🙂