Ibn Sina, a famous Muslim scientist, put two lambs in separate cages. Lambs were the same age and the same weight, and fed with the same food. All conditions were equal. However, he put a wolf in the third cage. Only one lamb could see the wolf but not the other lamb.
Months later, the lamb who saw the wolf was cranky, restless, poorly developing, and losing weight. The lamb died whilst the other lamb remained healthy. Although the wolf did nothing to the lamb next to it, the fear and stress that lamb lived in killed it prematurely, while the other lamb that did not see the wolf, was peaceful and developed well with a healthy weight gain.
In this experiment lbn Sına demonstrated the importance of mental health. Do not trouble yourself with unnecessary worry and concerns. Learn to rely on Allah and take care of your mental health.
"And for those who fear Allah, He always prepares a way out, and He provides for him from sources he never could imagine. And if anyone puts his trust in Allah, sufficient is Allah for him. For Allah will surely accomplish His purpose: verily, for all things has Allah appointed a due proportion."(Quran, 65: 2-3)
Source: Avicenna, "Concerning the Soul", in F. Rahman, Avicenna's Psychology: An English Translation of Kitab Al-Najat.