Is feeling ostentation during da'wah work, especially in front of girls, considered true ostentation, and what are the practical ways to get rid of it, and could its cause be a jinn possession, an evil eye, or envy?

As long as effort and diligence are present in purifying one's work for the sake of Allah and renewing the intention at its beginning, then any thoughts of showing off (riya') that may come afterwards are merely whispers from Satan to deter the Muslim from good. Therefore, one should not pay attention to them and continue with their virtuous deeds. If one senses a genuine, sudden onset of riya', they should repel it and renew their intention and sincerity. If the origin of the act was for Allah's sake, and then the intention of riya' suddenly occurred: if it was a fleeting thought that a person repelled, it does not harm him. But if he indulged in it, scholars have differed. Imam Ahmad and Ibn Jarir preferred the view that the act is not invalidated by it, and one is rewarded according to their initial intention in continuous acts like prayer, fasting, and Hajj. As for discontinuous acts like reading, remembrance, and spreading knowledge, they are interrupted by the sudden intention of riya' and require a renewal of intention. The feeling mentioned in the question may be merely a whisper (waswasah), and if there is any doubt about its connection to the evil eye, a ruqyah practitioner can be consulted.