Girlfriend rejected my proposal two years ago, now she proposed to me. How can I tell her I don't want to marry her anymore?

It's not because she rejected my proposal the first time, but because in the two years that followed I realized I don't want to tie myself to her specifically and legally. We can stay together but I will never marry her.

Girlfriend rejected my proposal two years ago, now she proposed to me. How can I tell her I don't want to marry her anymore?
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