The more I thought to do it seriously, the harder it became to contain my laughter.
When I tried to contain my laughter and to do it seriously, the harder it became to keep myself from laughing.
ご質問ありがとうございます。
① "The more I thought to do it seriously,"＝「真剣にやろうと思えば思うほど」
"the harder it became to contain my laughter."＝「自分の笑いを抑えるのが難しくなりました。」
② "When I tried to contain my laughter and to do it seriously,"＝「笑いを抑えて真剣にやろうとしたときに」
"the harder it became to keep myself from laughing."＝「笑わないようにするのが更に難しくなりました。」
ご参考に。
アメリカ合衆国 
