Firstly, and without wanting to make this sound like alcoholics anonymous, well done for being brave enough to admit that there is a problem and come here looking for advice. I of all people know that web forums can be a bit brutal on that front.
Secondly, how much of a factor would you say your dyslexia is? I can well imagine that it would damage your confidence. I assume it's been diagnosed by a doctor and so on. Have they been able to do anything about it? I know some people have some success with special types of glasses, with coloured filters and the like. Perhaps that's worth a look?
Thirdly, you are by no means alone. I've worked for a couple of firms of accountants in my time, and some of the stuff that gets sent out of the door by people is shocking. I've had to bite my tongue on occasion to stop myself from criticising the writing of people several grades above me.
How much do you read? I don't know how easy you find it, if your dyslexia gets in the way, but reading well written books is one of the best ways to learn good sentence structure, grammar and so on. It will also widen your vocabulary, which is a bonus. You don't need to be reading anything particularly high brow or intellectual, just so long as it is well edited. Try some of the big name authors in whatever field you're interested in - I'm sure someone in the Music, Books, etc forum can suggest some well written authors that you could try.
As mentioned above, Eats, Shoots and Leaves is a good book as far as a "crash course" in grammar goes. It's a long while since I read it, but as I recall it covers 5 or so key grammatical mistakes that people make that really wind people up. You might find that if you can get the hang of a few of the big mistakes then your writing will be more presentable, even if you're still struggling with other things. That should at least give you some breathing room at work if they're making a fuss about your writing. Just be warned that the book is a bit vigorous in its criticism at times, so don't read it if you're going to be offended that you're making the mistakes the writer is complaining about!
As has been said, proof-reading is very important. You'll probably be surprised at how helpful it is to read back through things that you have written and imagine what it would be like for someone else to read it.
I'd recommend starting with the basics, and getting those under your belt. Start with different types of words - adjectives, nouns, verbs, etc. Look at different tenses, look at verb endings. Get the hang of apostrophes as soon as possible, because that's a really obvious but simple thing to fix. Don't get bogged down with things like semi-colons or when to use whom, or stuff like that. 99% of people don't understand and don't care.
I'm losing my thread now, as a 4 year old has just climbed onto my knee.
If you want to post up particular things that were flagged in the report, feel free, and I'll see what I can do to explain them.
