If your daughter is now 19 the court no longer has the authority to address any issues with regard to your daughter. If your divorce decree provided that support continues, that would still give the court the ability to address any outstanding issues. You may want to talk with an attorney that specializes in disability law because these issues are very complicated and have many aspects that must be considered in deciding what is best for your daughter.
You may start a divorce action. You can then ask for a temporary hearing. A temporary hearing addresses issues that need to be resolved during the divorce proceeding. Your mortgage payment certainly falls within that need. The timing is important so seeking an experienced family attorney who can guide you through the necessary steps is critical.
You have two choices. One is to seek a modification of custody or a modification of the parenting schedule. Each has different standards to meet. It is much more difficult to modify custody. I suggest you seek out an experienced family law attorney who can inform you of the standards and whether you have enough to proceed ahead.
It is very difficult to get custody of your stepsons. I suggest you consult with an experienced family law attorney as there are many facts that must be explored to begin to answer this question.