、。
To be honest, I couldn't solve this problem.
Literal
True [possessive-の] thing [object-を] say [conditional-と], I [topic-は] this problem [object-を] solve-could not.
解けなかった ('couldn't solve') is the negative past of 解ける — the potential form of 解く ('to solve'). The 本当のことをいうと frame ('to tell the truth') prefaces an honest admission, softening what could otherwise sound like a flat confession of failure into a more confiding remark.