English is not my first language either, so I understand how difficult it can be to not only speak but read and write it fluently. There are things that I am still learning about it even though it has been my primary language since I was about six. It's actually my third language but my second spoken, lol. Sign Language is actually my first language and German my first spoken once I could hear.
What has helped me the most in improving in my use of tense, grammar and fluency is Schoolhouse Rock's Grammar Rock. It makes a lot of things easy to remember as well as understanding how to figure out sentence structure. You can watch the different grammar rock videos on youtube.
The songs and images make it easy to understand and remember. I often find myself humming or singing the songs while I'm writing. It may not fix everything, but it would probably help you as it has me. The other thing I use a lot to check myself is dictionary.com. no www at the front. If you are not certain about the proper tense of a word, check it with the dictionary and it will tell you how to spell it for past, present and future.
As for the nasty reviews, they are very hard to deal with. I have gotten ugly reviews on stories in the past (on a different site for a different fandom) that very nearly had me stop writing. The nice reviews were enough to keep me going but I do not allow anonymous reviews on any site any more. I do tell my readers that they can mail me or leave a note on my wall though. I have had several just send me a message. Maybe you could ask your anonymous reviewers to leave their thoughts on your profile instead?
I hope that it helps for what it's worth. Keep writing, and don't worry, things will get better.
~Starfyre