感到失望means feel disappointed. 可是每当他爬到一半儿时，就会由于劳累而摔下来。“ 一半儿”means on the half way. “时”means when or while. “会' means will or would. The meaning of the sentence is every time when he climb to the half way, he will fall down because of exhaustion.

"失望" - disappointed "一半儿时" - when half-way "会" - would/there's a tendency of

失望 = разочарованные 一半儿时 = на пол-дороге

对不起，云飞

云气，谢谢你！