You really need to realize that once a woman falls out of love--- it's unlikely it's coming back.
If there's something I learned the hard way (and also didn't listen to when I first came here... in 2008), it's this.
FWIW, OP, my first marriage was similar to yours - dating since we were teenagers, together 7 years, then married. Right around year 3 of the marriage, things really took a turn for the worse, but it lasted another 4 after that. But - same issue as you, just took her longer to get to the point that your wife has gotten to. Or more likely, that much longer for her to find a man she would leave for
This was pre-smartphones and pre-Facebook and social media, so I didn't catch her the way you caught your wife. It was only speculation for me. She had a good life at that point, so leaving would have been detrimental to her in many ways, so she stuck around, despite her no longer loving me. C'est la vie. Once she found someone she'd actually leave for, and could provide her with a similar lifestyle, she did. Be lucky your wife is being upfront about her feelings now, rather than waiting for wholly selfish reasons.
So consider yourself lucky in that respect - at least your wife is calling it now, rather than stringing you along for several more years. Years that I can't get back.
The hardest part for me (and it will be for you, if it isn't already) is that the person you've known for so long, and knew when you and they were so young, is rejecting you in this way. Rejection isn't easy regardless of age, or how many years you've known someone, but when the starting point was 15, or 16 or 17 years old, and you've subsequently gone through high school, prom, college, and starting careers and moving out - all with each other - it makes it that much more difficult. How could the person you've spent, essentially, half your life with do this to you? All those monumental life stages? All those 'new' things, and adventures, and future plans that will no longer happen?
And given that you two were 15 when you started dating, it's likely you had little or no experience with the opposite sex before each other. That makes it even worse. She's all you know. And unfortunately, she has the desire to know others.
And that was the gyst of my ex-wife's issues - she knew only me. She had other relationships before me, but nothing that lasted more than a few months. Typical teenage relationships. At the time we got together, I had dated somebody for 3 years, and had a few other short relationships afterwards, so I had "experience". She didn't have this. So when things occasionally get boring (as they inevitably do in long term relationships), the easy way out is to wonder what it's like with other people, and to start to fantasize about it.
My ex wife will hit this point again, if she hasn't already. AFAIK, she's still with the man she ultimately left me for. Things will get boring and stagnant there, too, and you settle into 'same old, same old'. But now she has experience with this, where she had none before. Much like where your wife is currently at.
In all honesty, your best and only hope is to talk to her about this in that way. My ex wife had a mid-life crisis of sorts at 30 - she even admitted that. Having really known only me and wondering if "this was it". After years and years of her and my friends (and siblings) starting new relationships, and the general excitement that goes with that, and the adventures they had, and the heartbreak and emotions, to come home to the same old, same old, day in, day out, same face, same routine - it's easy to start resenting someone, even blaming them for their stagnant life. And blame me she did. "You stole my 20's from me!" was a direct quote at one point.
So long (looong) story short - there's nothing you can do, or could have done, to avoid this. Very few people, IMO, are actually cut out for long term relationships when they start at such a young age. There's almost a biological urge to see what's out there, and otherwise get it out of your system before you settle down. It's hard to see the people you're around going through all the adventures (good and bad) while you've already settled down with someone, and life has become somewhat stagnant. It's hard to be 30 years old and every single adventure and story you have involves your partner.
Now, the girl I dated when we were teenagers for 3 years before I met my ex wife - we're now married. We came full circle. We broke up for the same reasons - 'what else is out there?' At the time, I didn't have the desire to know what else was out there, but she did. And ultimately, she was right. She knows what I went through with my ex wife, and we both realized that had we stayed together, it probably would have been the exact same scenario - one or both of us would have desired to know what else there was in life. So fortunately, we both were able to find that out for ourselves, and the answer (in our case, any way) was 'not much'. Of course we both had great experiences and adventures and we also had bad experiences and even traumas - but we learned things we otherwise wouldn't have, and we're both very thankful for that in the long run.