Your help is so enormous. Your proofreading is so great so great that I decided to learn by heart No, the sentence sounds odd. "Help" cannot be "enormous" You can say "You have been enormously helpful." Also, don't repeat "so great". This is something people do when speaking, but is totally unnecessary in writing. Finally, you say "that I decided to learn by heart" without saying what it is you will learn by heart. This cannot be omitted in English. So... You have been enormously helpful. Your proofreading is so great that I have decided to learn the text you corrected by heart.

The other answers are already excellent. I would just like to point out that a kind person or a teacher who corrects your work is not your proofreader, and it would be extremely rude to refer to what they have done for you as "proofreading". You should just call the corrections "corrections".

Your sentence is not really natural. Maybe you could say..."You have been a huge help. I have decided to try to memorise your suggestions because they are so helpful." :)