It could be many things, so I will start with some obvious ones.
Espresso is bitter if the roast is too dark. I don't think your blend is too dark although I don't know what sequella, colombia brown or barbera are like.
Espresso is bitter if it is over extracted. Set you grind coarser. Pack your portafilter with 30 pounds of pressure with a decent tamper. Observe the espresso once you engaged the machine. The first few drips should start at anywhere from 4 to 8 seconds after you engaged the machine. The pour should be a thick oozing consistency. When the color turned lightish blond, stop the extraction. Total time should be anywhere from 20 seconds to 30 seconds. I'd go with 26 to 29 seconds.
Espresso is bitter if the water temperature is too hot. I believe you have a heat exchange machine. And if the machine is idled for more than 5 minutes, the water is too hot, and the hot water needs to be flushed before you make your espresso. Engage the button for about 5 seconds or until the boiling water is cleared - it makes an obvious boiling water sound. If you pushed the button with the portafilter in the group, remember to wipe it dry before putting freshly grounded coffee in it.
On the subject of freshness, do you know how fresh are your beans? If you are in the US, those Italian beans are going to be pretty stale.
One suggestion for you. Find the best coffeehouse in your area, if the espresso is pretty good, ask the barista to stop by and see your set up and show you how to work the machine. You don't have to pay them anything, a couple of brewskies should do the trick.