I think, you can firstly buy a grammar book. When you are not sure about some grammar points, feel free to consult the grammar book. Secondly, do more exercises, because practice makes perfect. Some grammar points are not often used in daily life. You only have the chance to meet them in doing grammar exercises. Thirdly, you can take time to read more, listen more, so as to find out how native speakers choose the words they want to use, how native speakers construct a sentence.

Last but not least, you can search for help here at italki. There are many native speakers here, to whom you can turn for help. I hope that my suggestions can do a little bit help to you.