Being in a relationship and staying who you are isn't an easy task, believe me I know. Not only in romantic relationships is it difficult, but interacting with people, period. The more emotionally attached, the more we are affected. A common pitfall we find ourselves in is allowing another person's truth or view of reality to overshadow our own. In these cases we become powerless to the other person and start to only see their way. This is more than apparent in emotionally or physically abusive relationships where one person, through power and control, preys on someone with a weak sense of self.
If you find yourself or hear someone saying "I don't even know who I am anymore," I lose myself in relationships," or "where do I begin and you end?" you know it is indicating a loss of self and an over-identification of another's beliefs. More often than not it is a parent-child relationship, or romantic partnership relationship where there is a need to be loved. Be aware of the emotion "resentment", as it also indicates this type of relationship is present. Resentment comes when too much power is given to the other person and we don't listen to ourselves.
Here are some important things to remember. No matter what you are still you despite the kind of relationship you are in. You are of equal importance to ANYONE standing in front of you. You bring your unique perspective, experiences, and passions that offer a different way of seeing the world and that is everything. There are people who only think from their perspective and are incapable of really hearing and seeing you, which is frustrating, but at the same time knowing this can help you to let go of the need for their approval or understanding. Own who you are, your weaknesses, your strengths, and never give up.