Ahsanallahu ilaykum. A student has big dreams to become a da'ee and to teach due to the state of his people nowadays. But, he struggles with sins that may lead to major sins. Due to this, his memorisation and knowledge weakness. He is unable to fast due to his work that is physically demanding.

He needs to change his mindset. Even if he has fallen into sins, he should repent to Allah and not convince himself that his understanding and memorization are weak and so on. He needs to train his memorization by memorizing more often, attend lessons, and test his knowledge and understanding. If he wants to develop himself more and more as a student, what he should do is continue seeking knowledge and persevering upon this path for years.