I know I shouldn't scratch the scab on my arm, but the more I think about it, the more I want to do it.
I know I'm not supposed to scratch where it's itchy, but the longer I think about it, the more I feel like I have to.
ーI know I shouldn't scratch the scab on my arm, but the more I think about it, the more I want to do it.
「腕のかさぶたを掻いちゃいけないのはわかるんだけど、（掻いちゃいけないと）思えば思うほど、掻きたくなる」
scratch the scab で「かさぶたを掻く」
the more I think about it, the more I want to do it で「（掻いちゃいけないと）思えば思うほど、掻きたくなる」と言えます。
ーI know I'm not supposed to scratch where it's itchy, but the longer I think about it, the more I feel like I have to.
「かゆいところを掻くべきじゃないのはわかっているけど、（掻いちゃいけないと）思えば思うほど、掻きたくなる」
scratch where it's itchy「かゆい箇所を掻く」
the longer I think about it, the more I feel like I have to でも「（掻いちゃいけないと）思えば思うほど、掻きたくなる」を表現できます。
ここでは the 比較級 + the 比較級「〜すればするほど」の構文を使って表現できます。
ご参考まで！
カナダ 
