How can remembering a sin and taking pleasure in it, while striving against oneself not to commit it, be a sign of unaccepted repentance, especially if the person has completely abandoned the sin but it still comes to mind in their imagination?

Repentance has conditions: abstaining from the sin, regretting having committed it, and resolving not to return to it. If the sin relates to the right of another human being, then it is a condition to restore the right to them or seek their forgiveness. Repentance is not valid if one wishes to return to the sin and takes pleasure in recalling it. However, if repentance fulfills its conditions, and then the servant wishes to fall into the sin and takes pleasure in recalling it, this does not invalidate their previous repentance, and the servant is rewarded if they strive against themselves and resist this desire. Moreover, striving against oneself, examining the state of the heart, and repelling evil thoughts are very important, because the root of all corruption comes from the thoughts which are the seed of Satan in the heart, and which develop into intentions and resolves, and then bear fruit as actions.