Then you need to give SPECIFICS.
In order to change custody in almost every state, one has to prove that there has been a substantial change in the circumstances of the CUSTODIAN and/or the CHILD/REN. THEN one has to prove that the best interests of the children will be best served by changing custody.
Google 'Best interest factors + texas + child custody'.
desires of the Child;
the emotional and physical needs of the child now and in the future;
the emotional and physical danger to the child now and in the future;
the parental abilities of the individuals seeking custody;
the programs available to assist these individuals to promote the best interest of the child;
the plans for the child by these individuals or by the agency seeking custody;
the stability of the home or proposed placement;
the acts or omissions of the parent which may indicate that the relationship is not a proper one; and any excuse for the acts or omissions of the parent.