oooh... that seems familiar..
You may not like my answer
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when i started reading the first post i thought something but now that I know you're only 23 it's quite different.
it's not like you guys started dating at 25. You guys were 14/15. Litterally she has no other experiences than you.
When I met my exgirlfriend, her father had just dumped her mother after 20 years of marriage and restarted a complete new life with another woman, he now even has new kids with her...
What I am saying is that it's not too much likely that you're gonna loose her because of your mistakes, but rather because eventually it would have to happen, especially knowing that you guys dated at 15 years old.
It's not only for her, it's also for you.
You'll have more chance finding another person who has matured and has fucked around already with different partners from 15 to 25, and now know she is ready for a true serious relationship, rather than keeping a girl who didn't really had the choice since it came early and got carried on for 8 years in a relationship. Now she's feeling like something new, which is totally normal. I would never say that if you were older but once again, it started early, so she and you need to experience other things.
I know half a dozen of other guys in my social circle in the same situation. I have always been wondering what's gonna happen to them when the cuttie they got into a relationship with at the same age as you did, decide to fly away...
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It doesn't matter who is this guy. What matters is what choice she is gonna make depending on how she feels. and she TOLD YOU about it. She could have kept it secret if she wanted to. She didn't.
"when it's over, it means it was already over a long time ago" .
EVEN if you manage to keep this asshole away from her, if she feels NOW like slowing going towards "novelty" for her life, the battle against losing her is a bit useless.
The best case for you is that you belong to these extremely rare people who found themselves very young and you are just going through a major obstacle. You might be right.
In this case doing stupid shit with another guy might make her realize what you mean to her "it's when you loose something that you realize how important it is"
But this case is less probable.
IF you are objective about your involvement, saying that you have always done the best to make her happy, than you have nothing more to do. You have fear of loosing her on your side.
Does she ? Looks like she is in dominant position right now. You are working your ass out for her, and you are the one afraid to loose her that much while she is ok saying "hey i met this guy at work and he is hitting on me right now" ...
this is my modest analyse of the situation according to the elements you have given me.
Despite all, I hope it turns out better than expected.