What do you do if your girls catches you texting another girl?

That depends on why he is texting as well as what he is texting. If it is just harmless and friends as well as he isn't lying about it then not to worry but if you find it is getting out of hand, flirtatious and he is hiding it then you should let him know how his behaviour makes you feel. If he does not respect your feelings it would be in your best interest to rethink your relationship altogether.