I'm having a shifting issue, and I am not sure if there is a problem with the bike, or if I am doing something wrong (which is very likely, as I am still a relative newbie and pretty much learned to ride a road bike via trial and error).

Most times the bike shifts smoothly. The current problem I'm having always happens in the same situation -- if I am going down a big hill and then right into a steep climb. There are two such places on my regular ride route, and it is happening which such frequency that I dread these hills as I approach them. I'll be coming down the descent in my highest gear. As I reach the bottom and start to climb, I attempt to shift the front gear from my biggest to my middle chain ring (with the goal being to then quickly go to the smallest), and it won't go. The shift lever clicks and the chain starts making noise, but will not go into the next ring. I have to jimmy the shift lever a couple times, and sometimes I need to also shift the back gears to get it to finally move over to the middle ring. In one place where this happens, the hill I am climbing starts out very steep, over 20% grade, and I am stuck in my highest gear as I start to climb. I am losing momentum rapidly, starting to struggle and to panic as I fool with my gears, and it throws the whole climb out of whack (the grade decreases quickly to the 7-10% range, but the hill continues for about a mile).

Any thoughts?