I don't want to quit now. It would be a waste, especially since I've been practicing the piano ever since I was little.
It would be a shame to stop now. I've worked so hard at it all my life.
ーI don't want to quit now. It would be a waste, especially since I've been practicing the piano ever since I was little.
「今さらやめたくない。特に幼い時からピアノをやっているから無駄になっちゃう。」
quit で「やめる」
waste で「無駄にする」
since I was little で「小さい頃から・幼い時から」
ーIt would be a shame to stop now. I've worked so hard at it all my life.
「今やめるのはもったいない。ずっと頑張ってきたから。」
It would be a shame to ... で「…するのはもったいない・残念だ」
work so hard で「一生懸命頑張る」
ご参考まで！
カナダ 
