I didn't improve [in English] as much as I thought.
「思ったより〜」は "~ as much as I thought." という形の文章になります。
「思ったより〜しない」は " I didn't X as much as I thought"
「思ったより〜(形容詞)は It's (比較級の形容詞) than I thought. 例: It's bigger than I thought.
「思ったより〜(もっと)できた」は I did X more than I thought。
「改善する」は to improve で、「思ったより〜しなかった」の文型に当てはめると、 I didn't improve [in English] as much as I thought と言います。