Ask Mr. Smarty Plants

You have several issues in this situation. The sand, pretty normal for East Texas, is more prone to erosion than some of the firmer limestone or clay soils. The deep shade under the live oak is going to discourage many ground covers. Oak trees have a tendency to allelopathy; that is, they emit chemical substances to inhibit the growth of competing plants beneath them. We're sorry your St. Augustine failed, but not real sorry, because it is a non-native grass that slurps water. Although it is a shade-tolerant grass, the various problems cited above were probably too much for it.

The best plants for preventing erosion are native grasses, with their fibrous roots that grab and hold the soil. Again, the shade of the tree is is not going to permit a number of grasses to flourish. There are some grasses that will tolerate shade, which we consider less than 2 hours of sun a day, but they are not turf grasses, and will not give you a lawn to play on. 

Let's consider a multi-task approach to this situation, beginning with the deepest shade under the tree. Your best bet there is a good quality shredded hardwood mulch that will protect the tree roots, and replace some of the soil lost to erosion. In fact, as the mulch decomposes, it will change the structure of the sandy soil to some extent, and over time should help with the erosion.  The mulch will also permit foot traffic and activity; some playgrounds use mulch in their traffic areas. Ranging out from there into the dappled shade near the edge of the canopy, we can recommend some grasses good in shade and some low ground covers. Again, these will not withstand a lot of foot traffic, but they won't need to be mowed, and perhaps mulch pathways can be laid down between the grasses. 

Shade Tolerant Native Grasses

Andropogon gerardii (big bluestem)

Andropogon virginicus (broomsedge bluestem)

Bouteloua curtipendula (sideoats grama)

Chasmanthium latifolium (Inland sea oats)

Panicum virgatum (switchgrass)

Schizachyrium scoparium (little bluestem)

Sorghastrum nutans (Indiangrass)

Tripsacum dactyloides (eastern gamagrass)

Groundcovers

Phlox divaricata (wild blue phlox)

Phlox pilosa (downy phlox)

Phyla nodiflora (turkey tangle fogfruit)

Dichondra carolinensis (Carolina ponysfoot)

Andropogon gerardii

Andropogon virginicus

Bouteloua curtipendula

Chasmanthium latifolium

Panicum virgatum

Schizachyrium scoparium

Sorghastrum nutans

Tripsacum dactyloides

Phlox divaricata

Phlox pilosa

Phyla nodiflora

Dichondra carolinensis