I understand your situation. It is unfortunate, but common.
There is no national law about child custody and child support. That is a state issue, and the laws of your state apply.
You may have heard the old saying that "anyone who represents himself has a fool for a client." That is true, but when you are poor and cannot get an attorney, then you may not have another choice.
If you represent yourself, it is called "Pro se." It will take a HUGE amount of work. The court will give you no sympathy. You must learn the law and apply it to your case. You must quote the law (called statutes or code) and you must quote previous cases, which will support your case. Just asking for what you want will not work.
Your case seems to involve Parent Alienation Syndrome, which is the phenomenon of one parent turning the child against the other parent. So Google "Parent Alienation," and research it in Lexis-Nexis (see below).
First you need to know how to use a computer. If you do not know how to use a computer, then go the city library and take a free course. And you have to own a computer and printer.
Second, go to the library at a local university or college. (The city library will not have what you need for this part.) Ask the librarian to sign into their computers as a guest. Access the Lexis-Nexis database and research child custody law. Read the law and download it onto a thumb drive (flash drive or USB drive), so you can take it home to your computer. You are particularly interested in the "Annotated" revised code (law). Annotated revised code means that underneath each law will be a summary of previous cases that apply to that law. Read the summaries and "cite" (quote and reference) the cases that support what you want. When you cite statutes and cases, they are called "authorities." You absolutely must cite authorities, otherwise you will probably lose right away.
Third, you have to read the local court rules (and maybe the rules of appellate procedure), so you will know how to file a motion and how the court system works. Get some copies of motions already filed in court by real attorneys so that you will know what a motion looks like. Since you have been through custody hearings, you may already have copies of motions and be familiar with court proceedings, but you must read and learn the court rules in detail or you will lose by making ignorant mistakes.
Fourth, if you lose, the court might make you pay for the attorney fees incurred by your Ex in defending against your action. That can be thousands of dollars. But, if you don't have the money anyway, it may be hard for them to collect it. But if your ex is vindictive, he can make your life miserable with his collection efforts.
Fifth, if you get food stamps or any public assistance, you can request "indigent" status in the court, which means that you are too poor to pay court fees, which may save you the filing fees (or maybe not). But it would be worth it to apply for food stamps, and then apply for indigent status. The court clerk should have a standard form to apply for indigent status.
Sixth, you indicated that your kid(s) might be over sixteen and free to make their own choice under Texas law. That would make things even more difficult.
Seventh, never agree to anything your ex's attorney proposes (never, never, never), unless he is just giving in and giving you exactly what you want. Anything else that he proposes will just be an attorney trick to trip you up on some court rule that you do not know about. Typically, you should oppose any of their proposals, just because they offered it. Always ask for their proposal in writing, and then for time to research anything they propose, before you agree to it.
Eight, think very carefully before you start this project. It will chew up all your spare time, if you have any. Also, the court case may take two or more years and the kid(s) may be 18 by then, which will make your case moot. (Moot is also a legal term. but it means the same thing as you will find in any dictionary.) As soon as your case is moot, the court will throw it out. Maybe it is better to just try to maintain whatever contact you can with your alienated children and hope to re-establish better relationships when they are older and wiser.
