I see what you mean and I don't envy you. Did you read the "manage grazing" article?
Strip grazing sounds reasonable enough for summer. You can also have your grass tested for NSC, so you know what you're dealing with for a start.
Also, having your horse(s) tested for Insuline Resistance is a good idea.
And Google "magnesium for horses elektralife", that's the article I was talking about earlier, can't post a PDF with this phone, sorry.
It is manageable, it's just a matter of knowing how