At 31”, it's possible you could be drawing your 58” longbow beyond the draw length it was designed for. I know people are designing short longbows that are capable of doing things we didn't dream of a few years ago, but there is a limit to everything.
When you exceed the design specs on a bow, you draw into a range called “stacking” where each increment of draw length adds a disproportionate amount of draw weight. It is difficult to shoot accurately if you are drawing into this range, because draw weight may vary excessively each time you shoot the bow.
Adding to the problem is that drawing a 58” longbow to 31” creates a fairly sharp angle in the string where you hold it with your fingers. This is called “finger pinch” and makes it difficult to get a clean release.