Please help me dial in my espresso.

I'm relatively new to the espresso game and my espresso always tastes a bit sour... please help! I'm using a Breville Barista Express. From what I understand, the sourness is generally because of under extraction. The water seems to be at around 190-200 degrees, so I don't think that's the issue. I've tried to dial in the grind size so that pulling a shot takes around 25-30 seconds. At that grind size, the pressure gauge is at the very top of the range. I believe if I go finer with the grind, then it'll take longer to brew and push the pressure up higher. I'm not sure exactly what to do to address this. Could it be that I'm tamping too hard? I push relatively firm, aiming for about 30 lbs of pressure. Thanks for the help!