My growth has been slower than I thought it would be.
ご質問ありがとうございます。
「思っていたよりも」＝「more than I thought I would」
「自分の成長が遅い」＝「my growth is slow」
このように訳したら、「My growth has been slower than I thought it would be.」が良いです。
でも、その代わりに「I haven't improved as much as I thought I would.」と言う会話的な表現も全然大丈夫です。
因みに「英会話」は英語で「English conversation classes」に訳すのは普通です。
例文：
I've been taking English conversation classes for over a year, but I haven't improved as much as I thought I would.
英会話を一年以上続けていますが、思ったよりも自分の成長が遅い。
ご参考になれば幸いです。
"My progress has been slower than I thought it would be."
"I haven't improved as much as I thought I would."
"My progress has been slower than I thought it would be."
"my progress" 「自分の成長」・「私の成長」
"has been slower than ~" 「〜より遅かった」
"I thought it would be" 「思っていたより」
"I haven't improved as much as I thought I would."
"I haven't improved" 「私は成長しなかった」
"as much as I thought I would" 「思っていたのほど」
アメリカ合衆国 
オーストラリア 
