I feel sorry for you, Jemmyjem, because I know how wonderful it is when your kids get married and have children at what you also feel is the right time for them, and you can just enjoy it wholeheartedly with them.
I deprived my own mother of this delight by doing something similar to your daughter, but younger, and I feel very sorry about it, for her.
However, it is your daughter's life and tbh, whether she gets married or not isn't actually that important imo. What really will change her life is if she has children at a young age, before she qualifies, for example. And even then it's not the end of the world - she would get a child and you would get a lovely grandchild.
There is a lot that is really great about your daughter's situation. Her boyfriend is the same age and it sounds like they have the maturity to keep a relationship going at a distance. For the last two years, at a time when other parents worry about their daughters travelling abroad alone, meeting unsuitable men, being in danger, you have known that she is protected by her friendship with this boy. They will grow up together and they will have their friendship to cushion them against whatever life throws at them.
I think the only thing you can do is try to think positively about it, for your own sake. Your daughter is being unfair expecting you to be overjoyed, and I wouldn't hide your feelings from her, but nor would I refer to them again unless she does. I would stay in the role that she has put you in and go through the motions as required, with as good a grace as you can muster. And if you feel yourself getting caught up in the excitement of it all, I would relax and enjoy that too, without thinking too much about the future.
I hope you have a partner who you can confide in, or a friend, because that helps too. And of all the awful things our children can do to us, this is not the worst, not by a long chalk!