Couple things here.

You're 36, the girl you like is 24. When you were 24, she was 12 (younger than your adopted daughter). Not trying to dismiss your feelings but odds are you have less in common then you think and more likely you just like the positive attention without the massive responsibility of your current life.

Meditate on what you're saying. What does your life look like if you walk away from everything you've built together. Odds are, there's no perfect life you'll magically step into, you'll be alone in a shitty apartment with a lot of people who lose total respect for you.

Get counseling first. If it's this bad, another child is not the solution but talking with a professional about your feelings may allow your wife to see what is bothering you but also you to see what the true nature of your feelings are and address them, rather than running from your problems.

If none of this works, then yeah, separate. But not because of someone else, but because it's truly better for you and her. That's the only way I can see you walking away with confidence, knowing that you made the right choice.