If somebody has started encroaching upon your mental space, if somebody has really started affecting the way you go about your day-to-day activities—the other person has started showing up his presence even where he should not be—then you should know that the relationship is not going in a healthy way. You should know where to draw the line.
Similarly, you have to see whether a particular angle of domination is creeping into the relationship. Our unhealthy tendencies show up in the most pronounced way in our relationships, especially if the relationship has a romantic or a sexual character. Then all the garbage that otherwise keeps lying hidden within just shows up in all its crudeness and with all the stink.