I gave her what she wanted and willing to change for the better, why can't she accept me back?

I made a mistake taking my then girlfriend of 5 years (now ex girlfriend) for granted for too long. The only reason I was stalling is because I was nervous about parenthood, don't know what's it really like raising your own child and whether I'll do a good job or not. I was given up for adoption as a baby and found out the truth during my late teen years. Well before discovering it, I always knew I didn't even look like my family (always had my suspicions of being abandoned) and other relatives treated me differently, as if I were an outcast. Till this day I don't know who my biological parents were. They signed a contract that they didn't want to ever be contacted by me.

She broke up on March but just last week, I went after her. I didn't call, text nor email her. For the first time in all these years, I showed up on her door, unannounced. I got there with everything to lose, nothing to gain but (I already lost her when she left) I didn't care anymore. She doesn't want me anymore. She told me it's best we go NC for good and how she no longer views any future with me. I popped the long awaited question she wanted to hear; I gave her what she wanted and now she declines me. She said no to my proposal.

Why won't she accept me back? I'm willing to change for the better, be the man she wanted. People make mistakes. Now I have to go back to the store (that's not even near) and get my refund on that diamond ring. It was worth $3,000. How can she go from wanting a future with me, wanting kids with me to suddenly wanting nothing from me? This is a radical change.

I gave her what she wanted and willing to change for the better, why can't she accept me back?
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