Is it sufficient for me to intend in my heart to forgive those who wronged me, and to keep that a secret from them, in order to receive the reward of secret charity?

What you did was contrary to what is better. Therefore, your explicit declaration of forgiveness is important to show the purity of your heart towards your friends and your satisfaction with them. Your intention to forgive while keeping it secret does not fulfill this purpose, and your action might give the impression that you did not accept the apology of those who apologized to you.