I feel that way every time I achieve something. The feeling of "success" doesn't last forever... in fact it's quite short lived. And then you need to go and do something even better in order to redeem that feeling... but then it goes again and you need to do even better again.
I got into a top RG uni too, felt great for a few weeks, then it went away. Then I finished top of the year in my class, and that felt great too... did that every year. Then I got a first, and that felt great, but then the feeling went away. Then I got into another RG uni for a PhD, which felt great, and than the feeling of success went away.
You constantly need to "level up" to maintain the feeling of success. It's almost a drug.
But yes, once you achieve something that you work long and hard for, and spend weeks/months/years planning and contemplating, it becomes very old very quick once you actually achieve it, I feel. Which is completely normal.
I think we place too much emphasis and over-estimate how something will change our lives. For example, when I think about my next big goal (completing the PhD), I have an image in my mind of how life will be when that's done. I'll wake up every morning and acknowledge that I'm "Dr." me, and that I've proved myself to be extremely intelligent and well-qualified, or whatever.
But actually, I'll probably wake up the day after my PhD feeling exactly like I was when I woke up the day before I got my PhD. Yes, things will be different, but the world won't suddenly become a shade more rosey. It won't be as big a deal as it was before I'd started my PhD, because it's something I'll be working towards and expecting to get for 3 years.