hey greeny,
welcome to the site and thanks for your question!
It must be hard to know a lot of words and not be able to spell them. I assume that you have no trouble with the same words for speaking or listening? That would mean that you need to try to do more reading and writing. The reading will help you see the words as they look written down and help you recognise them and the writing of the words yourself will give you the practice you need.
I would suggest you make the reading and writing fun for yourself to start with and read things that you are interested in, which are graded to your level of English. You should also try to write the words which you learn as new vocabulary down. This writing could be only short sentences but make them fun and maybe you can put a few sentences into some paragraph about some topic which you like.
I have the same problem in Khmer, I can speak and listen okay, but I cannot read and write, so I do understand your problem about spelling as a second language learner of the Khmer language.
Hope this helps you a little bit,
have fun,
Paul :wink: