mentally I can't stop because I'm starting to feel like I've made the biggest mistake of my life by letting the most important thing of my life slip away...
You went one way, she went another, she hardly slopped away as you so romantically put it, because you had no control over her actions, and still don't.
While its understandable to have those feelings, even natural for humans, it would be absurd, and stupid to act on them.
The feeling will pass if you let them, or distract you, and then you will start justifying those feeling by finding faults in your partner, and that is not healthy, realistic, or fair.
Better to cope with your feeling, and deal with your reality, as I have those feelings too, from time to time (when the wife pizzed me of) but be darned if I'm going to go for the birds in the bush, for the one in the hand.
By the way I still have fond memories of high school sweethearts, but after more than 30 years of marriage, that's all they are, old feelings, and memories.