Hello dear,
First of all, remove all myths & misconceptions from your mind about night fall.
In fact, it is a normal physiological process & doesn't have any bad effect to the body and mind.
Since you remain disturbed because of your past experiences, you can try out the following measures:
1. Improve your social life by spending more time with friends and relatives.
2. Keep your mind active and busy at all times either through curricular or extra-curricular activities.
3. Have a diet rich in fruits, vegetables, fish, nuts & honey.
4. Avoid fast foods & lipid containing diet.
5. Avoid
smoking & alcohol.
6. Keep away
stress, think positive.
7. Exercise regularly & practice meditation
These measures will be helpful in improving concentration & builds up the confidence level.
Take care.