Another variable to keep in mind is strength. Regardless of what the real flavor balance of the shot is, if the shot is too intense (too high a brew ratio) you will perceive it as "bitter". Different people have different thresholds for what is too strong for them to distinguish flavors anymore. You can test for this by simply adding a small amount of hot water to your shot to see if that changes the perceived flavor.

If even the diluted shot tastes bitter, you truly are over extracted, but over extraction is actually pretty hard to do.

Guys, your advice really helped. Yesterday, first I tried 16gr dose. It gave me 35second 28gr shot, that was a little bit aggressive, but balanced! I then tried 15gr, without changing grind, and it gave me about 28 second 26gr shot that was very good, with more crema .