Married but platonically attracted to a much younger girl

I’m (31M) married to the perfect woman. We’ve been together for more than 10 years. I’m NOT saying this just for the sake of it. I truly love her and she will always be the love of my life. But I’ve been fighting with a feeling that simply feels wrong, and I need to hear some advice.

I’m attracted to a much much younger girl (18F). Let me be absoloutely clear: it’s nothing sexual. I’m pretty sure it’s not romantic either. I guess it’s platonic – but it feels kind of wrong?

I know her from horseback riding. We ride together, and I see youth and innocence in her, and the way she is passionate and excited about life and animals, the way she smiles with her whole being… I feel like I want to hold her and protect her from the world, from ever growing up. I want to put my big adult’s mask aside and just enjoy the beauty of life with her, in nature, and forget about everything else. I know lots of people and she’s absoloutely unique.

Of course, I know this is unfair to my wife. I go through EVERYTHING with my wife – both the good AND the bad. With this girl, it’s only the good, the happy moments at sunset. I know this, and that’s why I’m married to my wife and will always be with her and anything else is out of question.

But I have feelings for this girl and I can’t shake them away. It’s definitely love. It’s not sexual. It’s not romantic. Almost as if she were my little sister and I just wanted to hug her, see her smile, protect her from the world… If you’ve read the catcher in the rye, that’s exactly how I feel.

I think she sees me in the same way. She has all her teenager boys drama; it’s not like she’s romantically in love with me. But she does love me too in a way. Does this make sense?

What I struggle with: thinking about her… I feel the damn butterflies. I just need to know whether this is fine and everything can remain innocent and platonic, or whether this is a trap and I should slowly get away. I just need a certainty, but my feelings don’t seem to let me think clearly. In one way, I feel like it’s society fault, like it’s black and white, like you’re only supposed to love one person in your whole life and that’s it. Well, I don’t think it’s that simple.

My wife knows this girl and I are very close, but I haven’t fully opened up about the depth of my feelings because the risk of being misunderstood is simply too high, especially when there’s nothing going on and will never be.

Anyways, has anyone ever gone through anything like that? Am I overreacting? What are your thoughts?

TL;DR: I’ve been married for 10 years but feel platonically attracted to a much younger girl. Almost like a little sister. In one way I feel this is wrong; in another way I feel like it’s genuine and right.