Is there any consequence for me because I broke my promise to Allah not to return to masturbation, given that I went back to it a week after repenting?

You must repent to Allah and abandon this forbidden act. Nothing more than repentance is required of you, and the words you uttered to yourself have no consequence, for an oath and a vow are not binding except by uttering them with the tongue. And these words—even if you had uttered them with your tongue—would not necessitate anything.