One of the hard things in espresso preparation is adjusting the dose and grind based on taste. I found this, at least for me, to be hard for quite some time.
But, after making and tasting thousands of shots, I think that in most cases when we do not like flavor, we are actually under-extracting. So if your shots is excessively bitter or excessively sour, in all likelihood, you are under-extracting and need to grind finer and use lower dose to increase extraction.
But espresso preparation and our taste is not that linear so if that does not work for you go in opposite direction ;-) Increase dose and grind coarser.