「」。
It's painful to go through life with a 'failure' label slapped on yourself.
Literal
Self [on-に] 'failure' [attributive-の] label [object-を] affix-[and-て] live-[continuing-ていく] [nominalizer-の] [topic-は] painful [thing-こと] is-[copula-だ].
レッテルを貼る ('to affix a label, to slap a label on') is a set phrase of Dutch/Portuguese origin — レッテル is from 'letter' (the Dutch-era loanword for 'label'), and the whole phrase is used figuratively for categorizing or stereotyping someone. できそこない literally 'cannot-be-well-done,' meaning 'failure, reject, defective product.' ~ていく ('continue onward') emphasizes the prolonged suffering of living like this.