The TX CHL shooting test is, in my opinion, more to give the instructor a chance to observe you with a firearm to make sure you're not a danger to yourself or others than it is a real shooting test.
If you have a working handgun and still fail the test, then, without a doubt, you have no business carrying a handgun. In other words, don't worry about the shooting test, it's not something to be nervous about and you won't need to practice for it unless you've never shot before.
As far as the written test goes, they're not trying to prevent people from getting their CHLs, they just want to make sure you paid attention in the class. You will pass the written test with no problem.