Dear Snailtwist:
It is clear to me, following reading your post, that re-homing your dog is the right choice for you. Let the dog’s future owner know of him attacking another dog, a friendly dog in the park, so that the dog can be trained so to prevent such from occurring again.
You had best intentions when getting this dog and you invested a lot in it but it didn’t work out for you. The experience with the dog is hurting you, so find him another home.
In life we experiment, try new things. It is not you who failed, it is the experiment (owning a dog, in your case) that failed. It is the same for everyone: some new experiences benefit us and some new experiences don’t. If you learn from your experiments/ experiences, you don’t fail.
anita