First you need to deal with a jurisdiction issue,
Others here are more well versed in it, but if the originally filing was in Georgia, and ex has never moved for the juridsiction to move to Texas, then Georgia still has control of the court issues. However they may move it to the Texas courts if ex request it as that is where the children are residing.
2nd. you should start by filing contempt on the visitation.
You will need to docuement everything well. Sparc (www.deltabravo.net) has sample letters to use. 1st you notify ex of your intent to exercise visitation per the order. Then if denied you document. After a few, you would want to file for a contempt charge. Many states have the forms to allow you to do this yourself, but you should probably retain an attorney.