I'm noticing one thing- the beginning does feel 'secretive', like I am meant to be caught by mystery. I don't know quite what's happening. I do feel the power of the description, and it bodes well- I like the tone of this Princess Celestia very much. These are seriously royal Princesses, and that rings true for me.
My concern is simply that, to get a killer start for a book (and I'm not saying I always manage this myself!) you've got to anchor the reader in the known, and fill them with specific questions of what it might mean, where it might lead. I'm seeing a dramatic scene I don't understand. It raises questions but I have no context to place them in, and then raises even more questions until I'm reeling with confusion…
…and then the ending. Chills. Yes, you should be writing this.
Two points, one tiny, one larger- you don't need to adopt either, but these are my thoughts.
One: "watching her step through a portal of pure magic into a realm of pure magic" isn't great because you're repeating 'pure magic'. I'd just say 'step through a portal into'.
"Many generations ago, the Goddess of Love realized that she had forgotten love. She loved all the little ponies, of course, and she loved the land and its gifts, but secretly she longed to know love of the kind that the little ones felt for each other. She had never been a nursing foal, weak and utterly dependent upon love. Her body had never coursed with the lusty, foolish energy of youth. And, of course, she would never die in the arms of a lifelong mate."
This is your preamble. You've got to telegraph that much. Don't remove it from where it is- that's the moment that it all comes together, the moment I realized what you were doing. I should never have been sitting there frustrated and confused leading up to that point. It's the beginning of a repeating story. Put it into italics, like an epigraph leading off EVERYTHING, and throw those words up first.
This will do two things. It'll give just enough context that we know it's the Goddess of Love doing a desperate thing. It is not an evil alicorn being destroyed, not a villain being vanquished. Arguably Celestia and Luna are not 'Goddesses' of sun and moon, but that's a lesser issue and you can't very well call her 'the alicorn of love', what you have feels right.
The far more important thing it'll do is alert the reader, sensitize them when you KNOW you're about to leave them stunned. Use it in italics with an ellipsis… to show that the story is trailing off. Tell all the rest of it, just as you've done- I have no real gripe with any of it, just that it's holding back so much. Then, when we reach the end section, and those words happen again, but they go ON and we hear the rest of it? I guarantee that it's okay to telegraph that it's about Goddess of Love, it's okay to allow readers to guess that maybe it's something to do with Cadance and so on- because they must read your words to have their suspicions confirmed, and they MUST wonder these things more directly, not be distracted by a wider range of possibility.
And then, when they finish the chapter, they'll be yours.
As I am. *doffs hat* Write this, Tactical. This is god-damned wonderful.
Are you okay with the fact that ponies- myself included- will want to see the story begin anew somehow? With another alicorn born? And to follow the new life? It's okay if you can't do that, but this is just stupidly good and you might really need to not let go of it so easily. The main problem is that you'd immediately move beyond FiM era into the near or distant future, with the only remaining characters the Princesses. But you've got 'em so good, and it suggests the story would revolve around them and their relationship with the next 'mortal alicorn of love'…