I don't know how we can separate the two. Google books suggests Lactuca graminifolia is more common in Texas although both are present. (page 114).
Photo of leaves on page 413 show deeply cut lobed leaves, unlike those on your plant Christine.
https://books.google.co.uk/boo...
The clue could be in the flowering period. When did your plant flower? Page 81 ..
https://books.google.co.uk/boo...
L. graminifolia flowers in early spring. L. canadensis flowers in summer.
