Is taking back my right from the oppressor considered a sin and a breach of an oath, necessitating expiation, after having previously forgiven and called upon God and His angels to witness it?

You have done well by pardoning those who wronged you in the past. Your reward is with Allah, and your right to claim recompense from them is forfeited. However, pardoning someone who will wrong you in the future is not permissible, because it is a relinquishment of a right that has not yet become due. Therefore, you can claim your right from someone who wrongs you in the future.