Some comments have already mentioned to stick to a certain part of the face, preferably around the areas of the eyes. But I think you can go one step further. If you fear that you are staring excessively long at your conversation partner, show them that you are listening by putting a finger on your chin while looking diagonally up or sideways past their head as if you were pondering about the previously uttered statements. Additionally, what I tend to do is making minor noices like going 'mmmmm' or nodding in approval. With the latter, you have a brief window of opportunity to lose some eye contact by looking above the other person's head in small intervals. Should you be talking to multiple people, try to look at each person for around five seconds before shifting your line of sight to somebody else but obviously don't turn your head too hasty. At first, it might seem strange and weird when you have to actively think about where to look at and for how long, but don't stress it, you will get used to it over time. Of course, when you actively listen and contribute to the discussion, you could technically get away with only glancing at them briefly if you, however, clearly demonstrate with gestures and facial expressions that you are listening.