I know just what you mean -it's terrible to not have a good comprehension of the grace of God in such matters as sex and "lust", especially when one is young and having to deal with the subject a lot.
It seems to me it's because so few preachers and so-called Bible teachers are teaching the grace of God and basic Christian morality -that we are saved by faith in God's grace, and our righteousness comes from that faith, not from what we do or don't do. My life would have been so much happier if I'd understood that from the start.
In the earlier pages we talked about how "lust" is only immoral when it's covetousness, in contradiction to the tenth commandment, not when it's simply desire, sexual or otherwise. Covetousness is not taking what's someone else's, which is of course, theft, but simply wanting it, whether another's spouse or house, and its cure is contentment with what one has.
It's interesting to compare the NASB version of these verses from 2 John with the new ISB, as to "lust"
NASB: For all that is in the world, the lust of the flesh and the lust of the eyes and the boastful pride of life, is not from the Father, but is from the world.
ISB: 16For everything that is in the world —the desire for fleshly gratification, the desire for possessions, and worldly arrogance —is not from the Father but is from the world. 17And the world and its desires are fading away, but the person who does God's will remains forever.
This seems morally reasonable to me: that "earthly desires" aren't evil in themselves, but aren't what eternal life is about and are only mortal distractions.
And of course, "fleshly gratification" includes a lot more than just the desire for sex.
I hope that's helpful.