She left because she didn't get a proposal. How to get her back?

Anonymous
We dated since 16. She told me she wanted to be married by 28, I was working on making that happen. I wanted it to be perfect.

She left and immediately is with another guy. She says she is angry and frustrated that the relationship was not going anywhere. Should I just propose? Should I move on?

I love her with all my heart and seeing her with another guy is killing me, especially knowing that if I had have just popped the question she would still be with me.
Updates
+1 y
Do you guys think that this is a normal thing to happen in a relationship that started so young? I left the relationship for a bit when I was 20 and realized this was what I wanted. She never got that chance because she was hurt at the time I did.

Updates
+1 y
After exploring life without us what do you think the chances of her comming back is. I was good to her.
She left because she didn't get a proposal. How to get her back?
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