I liked one of my friends but then due this I affected my exams so just told him that we can't talk ..but then I txted him i miss you because I could not get over him is that right did I do something wrong??

The word that you need to get acquainted with is moderation, which means: The avoidance of excess or extremes, especially in one's behavior.

There is nothing wrong with keeping in touch with someone - texting, phone calls, emails. However, if you do this exclusively so that you forget to take care of the other things, then you've crossed the line.

Do things in moderation and yourself under control with some excellent time management skills and you can accomplish more than you think.

Set aside time for homework, for chores, for eating, for sleeping, for keeping in touch with people, for walking the dog if you have one, etc. This is a life skill that you will get better at with time and practice.