What should I do about the problem of not being able to forget a girl I admired at my previous job, despite my distance from her and my marriage, and the resulting increase in thinking about her and practicing masturbation?

If you are able to marry the one you have become attached to, then do so. Otherwise, sever your relationship with her, and beware of continuing with an attachment that could lead to infatuated love, which corrupts hearts. Masturbation is not permissible, especially for a married person, just as it is not permissible to imagine another woman during intercourse with one's wife. Adhering to the limits set by Allah and lowering the gaze from forbidden things are among the most important reasons for a husband's contentment with his wife, whereas unleashing the gaze makes a husband lose interest in his wife and opens the doors for Satan.