And there we were one day, sitting in the park, actually thinking we might have to give her up — a heart-wrenching prospect considering that she was already ours/one of us.

Then this one day, sitting in the park, we actually entertained the idea of taking her back/giving her up, but it was too painful (to even consider); she was already ours/one of us.

Then, sitting in the park one day, we seriously thought about giving her up, but our hearts ached at the thought — she was already ours/one of us.