I was embarrassed seeing my parents showing me off at the concert.
Literal
Parents [subject-が] concert [at-で] my thing [object-を] boast manner [object-を] see [and-て], awkward-was.
ばつが悪い 'awkward, embarrassed, feeling out of place.' 自慢するさま 'the manner/appearance of boasting' — さま 'appearance, way of doing.' A cringe-worthy but relatable family moment.