How can one differentiate between spiritual laxity in worship resulting from "for every deed there is an initial intense drive, and for every drive there is a period of slackness," and that resulting from sins and transgressions, and that resulting from the evil eye or envy?

Every action has a period of activity and a period of lethargy. So, whoever's period of lethargy leads him to the Sunnah of the Prophet, peace be upon him, is guided. And whoever's period of lethargy leads him to anything else has perished. A distinction can be made between lethargy in obedience caused by sins and lethargy limited to abandoning supererogatory acts. The first is lethargy away from the Sunnah, while the second is lethargy towards the Sunnah. Lethargy accompanied by signs of the evil eye and envy is caused by them.