Every time I see someone who looks like you, it makes my heart race and I get happy, but then I soon realize it's not you which makes me sad.
この場合、次のような言い方ができますよ。
ーEvery time I see someone who looks like you, it makes my heart race and I get happy, but then I soon realize it's not you which makes me sad.
「あなたに似た人を見かけるたびに、ドキッとして嬉しくなるけど、あなたでないとすぐに気がつき悲しくなります」
someone who looks like you で「あなたに似た人」
to make my heart race で「心臓がドキッとする」
ご参考まで！
カナダ 
