I used to live in the Houston area. I hated our high-e single-paned windows. Here are a few suggestions:
1) reflective window film/tint. It's fairly easy to install, and once installed, you'll never notice it again (except for the reduced heat).
2) external shades. This may be a bit hard for you, given how high the windows are from the ground
3) curtains--get the kind which will reflect the light back outside as much as possible, along with insulating the house. Or, get white blinds (to reflect the light) with curtains in front (for insulation)