Shifter Confusion - Driving in 3rd

I am writing this question in profound embarrassment realizing that this could go down as the most stupid auto thing I have done in my life. I am not a car person but that is really not an excuse. I purchased a new Scion TC in 2005. I had been driving an older car so the new automatic shifting module was a nice change. Not used to the new shifter, I now realized I was confused with the layout and operation of it. Alright, here goes - I have been operating the auto, positioning the shifter in 3rd rather than Drive - which are on the same line with theleft position 3rd and the right position, drive. So that means driving in 3 all the time, city and highway driving. The rpms would be between 3500-4000 on hwy. Functioning properly in drive they are now around 2500. I have 37000 miles on the car from the past two and half years.

It has made me sick inside with this realization.

Could someone please advise me on what this would do to the car mechanically and what I should do?Amazingly the car has functioned perfectly, no sounds or issues with the transmission/engine.

Please help a very embarrassed driver.

Thank you,

Sterling0909