A lot of people confuse bitter and sour and don't have a clear idea of the distinction. I think that most of us start out being uncertain about this. But of course you can train your palate with practice whether we are talking about espresso, wine or fine foods. As your palate becomes more refined, it is easier to distinguish more subtle sour or bitter notes in coffee. So don't give up yet.
As a start, you could take a grapefruit and taste it. The grapefruit will be sour. Now try a small piece of the rind, which will be bitter. Try this several times and notice the difference. Here are a few other sour & bitter foods.
sour: grapefruit, lemon, lime, sour-flavored candy, fermented dairy products
bitter: citrus peel, unsweetened chocolate, uncured olives, dandelion greens
As you pay more attention to espresso or food in general, the better you will get at discerning the differences. There is a great, if expensive aid to honing your sense of smell called Le Nez du Cafe
. This is a set of 36 distinctive aromas that you can compare to the espresso that you are tasting. Smell and taste are closely related. A lot of people will find the price extravagant, but you could do something similar at home by taking the time to taste or smell some common items such as vanilla, caramel, butter, roasted peanuts, freshly roasted coffee beans, cinnamon, garden peas, caraway seeds, etc. It sounds simple but if you pay attention and repeat it a few times it really does improve your ability to discern aromas and tastes. Take a look at a coffee tasting "wheel"
if you haven't seen one before.
Another exercise that is sometimes done in culinary schools is blindfolded tasting. Have someone feed you samples of various foods such as apple, onion, cream, etc. Try to identify what you are tasting. It's harder than you might think when you can't see the food and don't know what it is in advance. Even if the foods are not directly related to coffee, the exercise will improve your palate in general.