You're not alone.
One of my favorite resources is a book by Harry Shaw called Errors in English and How to Correct Them. If you're in doubt on whether to use who or whom or how to punctuate when using quotes or the difference between effect and affect, this book explains it in an easy to read way.
Sometimes, we need help though to get something fixed in our brain. I'll list a grammatical problem and then a site I recommend to students for help with that particular issue.
Adverbs - Been told you're overusing those "ly" words? This site is useful to paste your text in and have it show them to you. It also works on other grammar issues, such as passive verbs. http://editminion.com/
Future in the Past Tense - An example of this is "would get" versus "got." http://www.englishpage.com/verbpage/futureinpast.html
Passive Verbs - Been told to watch out for passive verbs ending in "ing?"
This article on the ICL website is helpful: http://www.institutechildrenslit.com/rx/ws05/goodverbs.shtml
This one explains the difference between active and passive: http://www.towson.edu/ows/activepass.htm
Run-on Sentences - I've got two helpful sites for this problem:
Sentence Fragments - This site offers a quiz.
Verb Tense Consistency - This educational site has a verb consistency test. You can answer the questions then check your answers which is pretty useful.
Which English is it? Spelling and grammar different from country to country. - Here is a wonderful chart that outlines the subtle differences between British, Canadian, and American English: http://www3.telus.net/linguisticsissues/BritishCanadianAmerican.htm
I know this isn't a complete list of grammar problems, but they are definitely ones I see commonly in student assignments.
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