One of the hardest people to deal with in a relationship is a narcissist. A narcissist is a person who only care much about themselves and does not care much about others. They are self-centred.
Nonetheless, a narcissistic disorder requires a person to see a mental health professional. However, you ought to know that a person can have characteristics of a narcissist but he or she doesn’t have the disorder.

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Consequently, there are ways in which you can deal with such kind of people in a relationship.
Do not fall for their tactics
As much as you can sympathise with them, do not fall for all their tactics. Narcissist tend to make you feel guilty even for their own mistake. Learn them and let them know you cannot fall for their tricks.
Accept them for who they are
As much as you accept them for who they are, always be careful as some would only use this to their advantage.

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One of the bold steps you can take is accepting that they are narcissist and you can do very little about it.
Do not be afraid to let them know when they hurt you
If you keep on pardoning them for hurting you, they will continue doing that for long. As soon as they make mistake or hurt you in any way, let them know that they have done something wrong.
Have boundaries between the two of you
Remember these are self-centered people. Consequently, you can always consider their feelings but it might be different when it comes to you. Besides that, let them know that they are not entitled to anything. Give them less power over you.
Nonetheless, if you are having someone who has a narcissistic characteristics, you can always leave when it affects your mental health. You are important.