This might be confusing because we've just seen that he only joined the Rebellion a few years before the Battle of Yavin. However, he was born in 33 BBY. At six years old, the year would've been 27 BBY. This is long before even the start of the Clone Wars (22 BBY). The events depicted in the flashbacks on Kenari are of Republic officers (not Imperial). Thus, he's referring to being in a hopeless fight since he was a child.