I suppose milestones in a person's life are something you look back on, and say - "yes, this sufficiently divides my life into two parts - before this, and after this." It's the milestones that are coming up, that you know you will have to look back on, and ascribe them some measure of importance, that really freak the shit out of me. It's like going to Disneyworld as a kid - you have all these dreams and imaginations about what it's going to be like, and a headful of anticipation as you get closer and closer to being there. And then you're there. Maybe you get swept up in the moment, maybe you don't - maybe it comes and goes. As you realize how much it isn't going to last, you can get caught up in the last remaining moments, break off from them early (and spare yourself the heartache, or whatever), or just distance yourself from the whole ordeal, and live the moment.
You still get to spend a whole lot more time on the experience later in life, trying to justify all the mistakes, explain all the falsehoods in it, reinterpreting it to suit your current mood, filing down the edges so it fits into your current map of your life, and the way the world works, and all that. You usually forget the really great parts of it - at least as far as concrete examples go, anyway - there remains a nebulous, epehemeral haze over the whole of the milestone that tries to keep you from remembering it quite right.
You know what the weirdest part of this is? I got started on this whole random train of thought because I saw the word "fulcrum" in a book.