First of all, manually shifting an automatic transmission is not exceptionally hard on the unit. In fact, many carmakers today offer some type of manual control for their automatics. Examples include VW's "Tiptronic" and Chrysler's "slap stick." These electronic control systems give the driver the option of manual up/downshifting, or allowing the powertrain control module to handle the shifting chores for them in a fully automatic mode. There are several fail-safes in the system, such as automatic downshifts if you forget to do so, but from a fun factor point of view, moving the shift lever into the manual slot and bumping the lever forward to upshift and snicking it back to downshift can add measurably to the enjoyment of the driving experience. In light of today's congestion and frustration, shifting the transmission manually might help restore some of the enthusiasm for driving.
Even though your automobile might not have specific manual control mode, I'm sure it does have the conventional "PRND2L" selector, allowing you to up and downshift manually. If you enjoy doing so, by all means shift manually.
Are manual downshifts harmful to the transmission or drivetrain? The PCM won't allow a premature downshift that would over-rev the engine, but obviously, banging the shifter into the next lower gear forcing the engine to rev to high rpm isn't doing anything much good. And remember, the PCM will downshift the transmission automatically when speed or rpm drop below a specific point, or you step on the throttle to accelerate -- so the downshift is going to happen, one way or the other.
If you're downshifting at the same speed and rpm you would choose to do so with a fully manual transmission and it adds to your pleasure in driving, then by all means continue to do so.