You have asked a good question. I have seen many students facing this problem. Samia, it is natural when we are used to of doing a particular thing we become confident enough. For example when we are expert in driving a vehicle we become so confident that sometimes we commit mistake and suffer an accident. Similarly, so is in studies, we don't care about learning new things due to the perception of being learned and knowledgeable and hence the accident of failure occurs. Always think yourself to be a learner not a learned. Changing this perception will change your attitude and habits Insha Allah.