In TX: I keep children 50% of time, could my child support be modified or lowered?

I live in TX, have been divorced for 4 years & have a 13.5 year old daughter & 17 year old son (he just turned 17). Right now I pay $1,800/month in child support plus $200 in medical insurance. Total $2,000. I've done the calculations & based upon my current salary, I could actually pay $2,033/month in child support to total $2,233.

The divorce decree states their mother as primary custodial parent and we to follow the TX mandated "visitation" schedule. BUT both kids actually live with my girlfriend & I at our rental house 3 nights a week. Every Sunday, Monday, & Thursday nights like clockwork. Sometimes my son stays 4 nights a week.

I take my daughter to school on Tuesday & Fridays, take her to/from gymnastics Monday & Thursdays (she has gymnastics 4 nights a week) so 1/2 of the time. I bought my son's car, pay for his insurance, pay for both their cell phone bills, pay for my daughter's gymnastics meets (their mother pays gymnastics tuition), and pay 1/2 of medical expenses.

Their mother is a nurse (started working while we were still married), who is also going back to school. I pay no alimony. She makes $45,000/year and her household bills are $1,750/month.