Among the suggestions already given on the self tensioning guy lines, there is only so much you can do to get a perfectly tight pitch on a straight cut tarp. Catenary cuts are preferable for that tight pitch. Nothing you can do about that with your tarp.
That being said, your tarp could be a candidate for a 2QZQ pole mod. This is designed to told your tarp out in the middle to help shed off snow load and provide some extra volume underneath the tarp. This is the same effect as adding panel pulls to help prevent the wind from pushing in the tarp so much.