What are the reasons for a persistent feeling of fear and unease after committing to prayer, and what should my friend do about her feelings towards her maternal cousin while being careful not to fall into what is forbidden?

To feel tranquility and reassurance, it is recommended to turn to Allah and strive to increase faith and rectify the heart, and that is through: Sincerity to Allah and truthfulness with Him, as Allah Almighty says: "So He knew what was in their hearts and sent down tranquility upon them." And firm belief in divine decree and contentment with Allah in every state, for contentment brings down tranquility through which one's state becomes sound, while discontent removes it. Likewise, frequent remembrance [dhikr] and gathering for it, for in the hadith: "No people gather to remember Allah Almighty but the angels surround them, and mercy covers them, and tranquility descends upon them, and Allah mentions them to those with Him."