Congratulations on your engagement, I'm sure you're going to have an amazing wedding. It will probably be one of the happiest days of your mothers life too. Maybe she was triggered by your announcement. She has those happy memories too of meeting your father, falling in love and marrying. Sadly she is without her husband and that cannot be easy. I think once the wedding is out of the way she might start to digest her emotions a bit better instead of them being kept alive at the moment, if you understand?
I feel for the both of yous. I think it's important you adjust your expectations of your mother. She can't just recovery for you to feel happier, don't you understand she would do it for herself if she was able to. She's not well, that's not your fault. You love her, it's perfectly normal your hurting inside. She probably can't say it from the place she is in emotionally right now but your happiness is important to her. The last thing any good parent wants is to be a burden on their children. I think you need to grieve the loss of that moment that all children want with their mother, it's not happening the way you want it, she's not sharing in your excitement, happy that you are getting married etc...that would make anyone sad and I'm sorry you are not getting that, just cry, let it out. Then ask yourself what you do have? Doesn't it mean the world to you that she is still with you and will be there on your big day? She's not going to be the picture of health you want but she will be there all the same.
Just go and see her before your hen night and make plans to see her after too for your own reassurance. Make it like it's her idea 'would you like me to come to see you (the day after the hen night' ?
Forget those comments about being selfish, that's ridiculous, she's your mother. Just think more about what she would want you do to and my guess is, be happy, worry less.