It is widely accepted that being someone we are not, is very uncomfortable. But it is important to first understand who we actually are. Richard Feynmann once said "What I cannot make I do not understand." To actually understand who we are, we need a process (life) that we actually can analyze. We need to be someone who can sustain the process and focus on aspects that are most dear to us simultaneously. Both of these sub-processes give us some understanding of ourselves and can help us in being ourselves. So there could be cases when you are not being yourself. For e.g. when the sustaining process is not interacting with the focusing process properly or when one of the processes is not functioning.
Based on the situations you are in you get leverage and help in running the two sub-processes. You are usually in the situations you choose to be. If you choose to be emotional you get leverage on the second sub-process. If you choose to be less emotional you get leverage on the first sub-process. But as I said before the leverage on one process could be nullified if the other process is not taken care of. The leverage you get is usually in the form of experience (learned concepts from the data we are exposed to). So for e.g. having a leverage emotionally means that you are very experienced in various kinds of emotions and actually (un-emotionally) use those experiences in focusing properly. But gaining experiences is not easy either. It is well know in the learning theory that even though we need on polynomial number of sample data for learning concepts (hypotheses) it's actually hard to find the concepts. Experiencing is a hard task and so is being yourself.