Too bitter coffee typically has a certain appearance: there are big patches of white foam in the floating oils in coffee. It is the result of over extracted coffee. Apart from too long brewing time, coffee bitterness may be caused by other factors. Firstly, it can be temperature when water is too hot. Another reasons are grinds, which are too small, and ratio: too much coffee in comparison with the amount of water.Here
you can learn more about bitterness in coffee and reasons why coffee tastes this way.
Remedy: carefully control brewing time of your espresso. It is recommended to do it for nearly 25-35 seconds. Keep watch on the shot as it comes through and when you see the coffee coming out in a light color, stop immediately. One more advice is to adjust grind size to a more coarse one. Too fine grind will make your coffee pour out much slower, this, in turn, over extracts coffee and makes it bitter.