Jordy,
Why not going to the Sun extension site (they publish Open Office) and install the free add-on that checks grammar in Open Office?
You will find it at:
http://extensions.services.openoffic...t/languagetool
Also, if I may suggest it, try to look at your potential to learn English grammar in a more positive light--you have learned a lot to be able to write as you do. Why not check into some of the free online tutors for English and learn just a little more. It can help you in all of life. Some of ESL classes are quite good and free, too.