Is the discontent and grumbling that afflicts me when researching jurisprudential matters, due to my feeling of hardship, which leads me to think that religion is not easy, a result of obsessive whispers (waswasa), pain, and anxiety, or is it an objection to Allah's decree? And is it considered disbelief (kufr)? And does the forbidden discontent last, or does it quickly fade?

What afflicts you is merely a whisper (waswasah) that will not harm you as long as you rid yourself of it, shun it, and turn away from it. This is not a dissatisfaction with Allah's decrees or a hatred for His law; rather, it is from the deception of Shaytan. Therefore, you must strive against these whispers and repel them from yourself, and know that they do not harm you nor affect the soundness of your faith.