Help with writing.

Autistic is I. And I fail badly when trying to write any sort of prose. That's prose as in writing, not some sort of prototype rose.

Anyway....I'm not looking for an app! But am looking for some sort of structured help with writing English text. I'm constantly rearranging sentences. I take things literally. Really literary, not just like the kids say. I have no idea about tenses. And am useless without a spell checker. I also add parts at the start and end of the sentence. Not a very useful autistic super power, but that's the one I've got.

For example:  The cat sat on the mat which was sat on by the cat.  Naturally, this is frustrating and adding further complexity to something that already is a very difficult thing (for me) to do. 

I'm looking for some sort of tool kit to help out.  I downloaded the entire internet and there doesn't appear to be much in the way of help for adults with autism with this matter.

Answers on a postcard, please...