"Following Christ" doesn't mean you never mess up. The characteristic of a "true Christian" should be that they are NEVER okay or comfortable with their sin, and they are constantly striving to turn away from it and overcome it. When a person is carrying on in sin, and justifying it and refusing to acknowledge it (ie, a porn user), that's when you can say that person is not following Christ.
An interesting idea to look at here is that of one's besetting sin. By definition a besetting sin is one that someone is so prone to that it is actually characteristic of them. I once took a course from Catholic Sister who was also a Jungian analyst. She suggested that the thing about one's besetting sin that makes it so hard to overcome is that one tends to think of it as a strength.

The things that we tend to think of when we think of sins are things that are destructive outward actions. One has to do a certain amount of mental gymnastics to deny that they are harmful. But besetting sins are deeply ingrained mindsets. It can take a lifetime to suss them out and maybe never...
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