Robert,
It's not a question of talent, it's technique. If you can speak you can tell a story. Putting it down in written words is were it gets a bit hairy. If it truely is a passion don't give up. Follow Mr Deans advice and redo this last assignment.
In fact I'd suggest getting a little pissed at him and show him exactly how well you can do!
There are a lot of simple Grammar and style books out there that are quite helpful. Muse's suggestion is the best though. Once you finish the story set it aside for a while then read it out loud. If that doesn't show you the tense errors it's because you are auto correcting what you read (Saying what you meant to write rather than what you wrote) If that's the problem get someone else to read it aloud to you.
I'm sure you can tell this story in a more authentic way than you've written it.
I'm sure you don't speak like that. Give it another try.
Mad Lews