Sorry but as i recall your ex apologised for the way you found out (your dd blurting it out to you). It seems to me he was considering your feelings on some level from what i recall of you saying about it previously, especially if he was putting of telling you for fear of upsetting you.
Are you going to tell your daughter not to talk to mummy about it? Do you want your dd keeping secrets from you? Do you want to spoil her excitement just because you can't handle it too well? If you don't want to hear about it are you going to tell your daughter that and blow your top at her when it gets too much for you? No you're not, because you're a good mum and you want your daughter to be happy. Come here and let the feelings out by all means, i think we can all understand how incredibly hard this is for you.
But...
This is about your daughter being a bridesmaid. If it was to anyone else you'd be thrilled for her and excited yourself no doubt. Don't focus on who is getting married, focus on the fact that your daughter is being made a fuss of and is going to have a wonderful time. Try and enjoy it for her even if you can't enjoy it with her. It will be painful inside but you've been through far worse than this in reality, even though it might not feel like that right now.
Your ex and his family are probably not saying to your daughter 'go home and tell mummy all about it'. I'm sure they are worried and hence why your ex was reluctant to tell you and then embarassed and apologised when your daughter blurted it out. His new partners family will have some idea of how you are feeling. But they can not tiptoe around you and leave their life on hold until you are ready to handle it. You wouldn't wait for your ex to be ok about it if you were in the same position. And it is a really good thing that his new family have accepted your daughter as part of it all and part of her dad's life, your daughters life will be much better for it.
If they rejected and excluded her and tried to get him to move on from his past and even to stop seeing his daughter (some people do try and do that) that would be far far worse and I'm sure you'd have something not too pleasant to say about that.
Your dd is part of her dad's life, and this is an important day for him. I'm afraid you don't come into it for them in reality, and you have to find a way to deal with it for your own sake and for that of your daughter.
Of course it does hurt, horribly so. But no it really isn't deliberate, that is just your own feelings. If it was deliberate it would be them telling you all about it and not just your daughter. Soon as you can accept that and let go of it the better you will feel and the quicker you can get on with your own life.
Make your own plans, keep yourself busy, make arrangements now with a friend to do something on the day of the wedding. Book a spa day or an activity you've always wanted to try.
And if you possibly can manage it try and find little positive things your daughter can do so she can be excited but not full in your face. And get her to draw pictures of her dress when she's excited and in your face about it and you can't handle it. This will distract her and give you time to breath. And then you can talk about what colour it is, does it have sparkly bits, is she having flowers or a basket to carry, what kind of shoes she is wearing etc. Distract yourself from the wedding, focus on your daughter being a bridesmaid.
Ask her dad to let you have a photo of her as a bridesmaid, just her on her own or with the other bridesmaids. Doesn't have to be an official one. Maybe give her a disposable camera too for the day so she can make her own record of it, but if you don't feel up to dealing with the photos ask her dad to get them printed and help her put the album together. Go and buy some confetti together. Small things (quite hard for you all the same) but they will help your daughter see you are pleased for her. This is a big day for your daughter, try and share in it on some level if you possibly can.
You will be ok
Gilly