I tend to hear lust defined as being sexual - deliberately imagining sexual things about another person. I think for females, this happens differently, and that if we tend to fantasize about men, then it is more emotional than sexual, like an imagined story of a relationship with a man - the whole relationship, not just the sexual aspect of it.
If one finds oneself entering into these detailed fantasies, is this lust? I know that when the thoughts slip into the mind and one is not really thinking about what is happening, then this is temptation and not sin. But how about when, once one is conscious of what is being fantasized, and one tends to pick these fantasies back up again where they left off the last time, is that a mortal sin? Sometimes I can pull myself from the thoughts, and I acknowledge and know that they are wrong, but I also admit that I enjoy entertaining these fantasies.
I have confessed this in the past, but the line between whether it is a mortal or venial sin is difficult to distinguish. Another thing to note is that I have been like this since I was probably 12 years old, easily writing fantasies in my head about my life with another person. So it has become habitual, although now it comes and goes, but I still can admit that I like to linger on these scenarios at times.