Whether you are a professional writer or just an enthusiastic amateur who would like to be a professional someday, it never hurts to try and improve your writing. There are a few things you can try that will help.
Make sure that you know the basics
To be a good writer, you need a good grasp of the basics of grammar. Computer programs can point out general mistakes, but they cannot entirely do your job for you.
In this regard, writing is not so different from sports: the more you practice, the better you will get. Even on days when you are not feeling particularly inspired, it is better to get something down on paper than nothing. In many cases, you can come back to that work later on and find something of value in there.
Try to establish a regular time to practice as this will help to keep you doing so regularly.
Learn to self-edit
Too many writers simply whip off a draft and then send it to the editor without having proofread it themselves first. That is lazy, and not fair to your editor. Besides, if you are looking for work, the draft you send out to prospective employers has to be perfect. While no one expects a person to catch every single mistake in their own writing, seeing yourself where you have gone wrong will train you not to do it again in future.
Very few writers became great overnight. The ones that truly reached the heights of acclaim and fame did so after years of practice. Accept that it will take you some time to truly attain the level of quality you aspire to. Do not be frustrated – just let this motivate you to try harder and spend more time in front of the keyboard.