In a nutshell codependency is trying to control something you cannot control.
1. Self Esteem
One person in the relationship can only feel good when they are liked by the other. This person cannot maintain positive self-esteem without their partners approval.
A person may put their needs last in the relationship, to the point of emotional and physical cost. This is extreme selflessness that can be justified by a codependent person as they have a strong desire to please their partner.
If a codependent person cannot get thier partner to see or do things their way, they will use anger to try and control a situation. This person will be able to boost low self-esteem if they can manipulate a situation to go their way. At nearly any cost.
People in co-dependent relationships can be perfectionists which mean they are overly critical of themselves. They will blame themselves for things that go wrong. The struggle to get the other to love them takes over their life. They may prefer to be in uncertain and dramatic relationships.
A codependent person will usually wish they had more self-respect; they don’t trust their own perceptions so may have a few trusted friends whom they are constantly asking for reassurance. With other friends they may act like things are totally fine. They can be embarrassed to confide in certain people.
The above signs must be seen often and in a pattern to qualify as co-dependent. If there are further concerns, a mental health professional can be very helpful in dealing with concerns of co-dependency.