I am finding this OP very sad tbh.
Each time, and especially for children we seem to have forgotten that is something new, each time will be a first time. Because whatever has happened before, it will be different this time again. You will be able to notice and marvel at something different and your dd will enjoy herself for different reasons.
So let's take the panto example, your dd saw her fav character at the panto for the first time. Great! but what about the next she goes? She will probably be able to understand more. She might not to be frighten by one of the character. She will start commenting at length on what is happening and is it right or do they do it wrong etc...
So how do you know the 'best' time, the one you shouldn't miss is the first time? What is telling you that that specific first time, she won't be frightened, indifferent, bored? But she would thoroughly enjoy her second and third time?
Because of that there is nothing to 'steal' because every time she does something there will be something amazing happening. She will be doing something in a slightly different way, in a new way. It will be first.
So, you won't be missing anything if you remember that each and every single time she does something with you, it will be first. Remember and marvel and what she does.
And then, there is the issue that these special 'first time' aren't yours. They are your dd. Yes I am sure you are happy, proud of her. You are delighted to see her doing that, see her eyes sparkling etc... but these are her experiences, not yours. She isn't there to make you proud or happy or delighted. She is here to make the most of her life and learn to be happy.
And as parents, we here to facilitate that. Just that.