transmition

Hi

My transmition is automatic, I found my self changing gears from "S" to drive "D" all the time, just like if I have a mechanical transmition, I use it to slow down the car from "D" to "S" so I don't have to apply the brakes to hard, I start on the lights with "S" and then I get into "D" when I feel the motor need it, and I feel the car has more power this way that if I let the transmition to shift gears on its own if I only use "D".

So here comes the questions: :)

It is bad for the transmition or the car to be changing gears all the time?

Can my transmition breaks if I slow down the car, going to lower gear so I don't have to apply the brakes to hard when I'm going 120 and a smart guy braked in front of me?

thanks

Alex