If it hadn't been for you, I wouldn't have made such progress with my English.
If you hadn't helped me as much as you did, I don't think my English would have improved the way it did.
ーIf it hadn't been for you, I wouldn't have made such progress with my English.
「あなたがいなければ、私の英語はこんなに上達しなかったでしょう」
to make progress with ... で「…が上達する・向上する」
ーIf you hadn't helped me as much as you did, I don't think my English would have improved the way it did.
「あなたがあれほど助けてくれなかったら、私の英語はこのように上達しなかったと思う」
to improve で「上達する」
ご参考まで！
Without your support, I couldn't have improved my English.
I couldn't have improved my English without your support.
Without your support, I wouldn't have been able to improve my English.
ご質問ありがとうございます。
おっしゃったように「Without your support, I couldn't have improved my English.」は全然大丈夫です。丁寧だし、文法的に間違っていません。
ほかにあるフレーズは「I couldn't have improved my English without your support.」や「Without your support, I wouldn't have been able to improve my English.」になります。
どれでも使っても、ニュアンスと意味が一緒です。
ご参考になれば幸いです。
カナダ 
アメリカ合衆国 
