Can the English teacher jump in? I may be too late but it's always good advice: read everything outloud and, in general, if you pause naturally more than twice when reading it will be a run-on. One idea per sentence or two conjoined/contrasted ideas only. Don't overthink. If you don't have the chops, faking a sophisticated sentence will out you in a second. But everyone enjoys an easy to read essay (he says, head in hands, two days before all of the marks for English 12 have to be in to the ministry).