My sense of self was deeply bound up with that project, so its failure was hard to accept.

bound 是动词 bind 的过去分词，bind 的意思是"捆绑、束缚"。 up 在这里是副词，表示"完全地、彻底地"。 with 是介词，表示"与...一起"。 bound up with 是一个固定短语，意思是"与...紧密相连"或"与...密不可分"，形容两者之间存在深刻联系或依赖关系。