how should I motivated himself

Motivation, as theory suggests, comes when you see an end goal/benefit that you will gain out of any task/activity that you are conducting.

A motivated candidate can definitely perform better in the entrance exam than a student, who doubts his/her own preparation. In order to stay motivated and positive, it is recommended that no new topic should be started before 2 months of Gate exam. If you start preparing an untouched topic, then this will result in wastage of time which can otherwise be used for revising already studied topics. Strengthen the topics that you already know. This will boost your self-esteem and confidence. Also, keep doing mock papers once you are done with your syllabus as this will provide the list of topics where you still need revision and it will also teach you how to manage time during your exam and you will be familiar with the exam pattern as well through this.

Hope this helps!!!