how do I save my lawn

My first thought: Get rid of the lawn and look into xeriscape landscaping. Lawns are not natural and just waste water, especially in climates like Texas. The concept of lawn was created by the aristocracy to prove they had so much land that they could use it as a play ground and not grow food on every available inch. Now we are slaves to the whole concept of a sterile green carpet.
Stepping off soapbox...
I would say you probably have a soil fungus that is killing the grass and any local garden center can sell you a fungicide. Just read all the warnings on the label and maybe even look up the active ingredient on the internet for human and animal hazards. Remember, whatever you put on your gardens you will end up drinking at some point, as it all goes into our drinking supply (OK I still had one foot on the soapbox).

I like the way you think, Green Witch.

I think we would make a cute couple Gus.