Admittedly, I could have tried harder, but I still don't think all this criticism is fair.

admittedly 是由动词 admit（承认）加上后缀 -ed 和 -ly 构成的副词。admit 意思是"承认"，-ed 表示被动或完成，-ly 将形容词变为副词。admittedly 表示"诚然、无可否认地"，用于承认某个事实或观点，通常用于句首引出让步性的陈述。