You should file a Motion to Determine Arrearage, and work out a settlement with the other parent. This way, while your credibility with the court is great, you can address the math, make sure payments have been applied correctly, and obtain credit for other things you have paid for the benefit of the child.
Note: All of your payments since the last child emancipated/graduated from high school have been applied first to interest, then to principal. The interest amount on any arrearage before January 1, 2002 was 12%. All arrearage accrued since then bears interest at 6%. See Texas Family Code 157.265.