A person can only ever have an understanding of their own emotions and not of others. Certainly with an experiential understanding. You can never know another persons feelings in this way.
One can see another person who looks sad and the person may even tell us that they feel sad. All one can do then is recall how they felt when they were sad and assume the other person is feeling the same way as they did. However that is an assumption that may or may not be true. We can never know.
When a therapist says to a client, “I understand what you are feeling”, that is not actually true. It is accurate to say, “You look sad to me, I understand how I feel when I am sad and I am assuming you are feeling the same which you may not be. I can never really understand your feeling of sadness.”
The google definition of empathy is
the ability to understand and share the feelings of another.
As we can see that definition is unattainable as we can never understand the feelings of another. We can only ever understand our own feelings and then assume they are the same as another persons. Empathy is really quite a self involved process that is about self and not the other.