YOU SAID:

I am doing the best I can

INTO JAPANESE

私ができる最善をやってる

BACK INTO ENGLISH

I'm doing the best I can do

INTO JAPANESE

できる最善をやってる

BACK INTO ENGLISH

I'm doing the best that you can

INTO JAPANESE

ことができます最善をやってる

BACK INTO ENGLISH

I'm doing the best that you can