Regarding co-dependence in relationships: I grew up as an only child in a slightly volatile household, and often felt that I was responsible for my parents happiness. I thought that as long I was a 'good child,' they would stop fighting and love me. As an adult, I find it hard to assert myself, because I feel my feelings are psychologically tied with my parents' — if that makes sense. I know humans are not 'separate' beings ultimately, but one’s individuality/ego is important too, right?
The ego is both friend and foe. We cannot give to God that which we do not, ourselves, possess. A paradox, certainly but such is the truth that God and truth lie at the center point of the opposites.
Thus, if you are hampered by past experiences from being able to distinguish the boundaries between your self and others, you will be continually tested until that becomes clear. If another person is domineering or demanding, you will perforce be required by the law of karma to find ways to stop the pendulum of passive-aggressive reaction.
Based on your statements, it seems the first stage of awakening is not to allow others to use you as a doormat (that may be too strong a statement, for I don’t know the particulars — but you state that you find it hard to assert yourself). This doesn’t mean shouting or being angry, but calmly responding (whether in the moment or later when the heat of the moment has past) to state what you feel is yours to do and that which isn’t yours to do or to accept from the other person.
In the beginning this assertion may be a little rough and result in some argument or tension. You yourself may find it challenging to remain calm. But if you intentions are good and aided by meditation and grace, with practice you will gradually be able to do so calmly.
It takes "two to tango." If you decide not "to play," it gradually becomes no fun for the other person to "push your buttons." This gives that person permission to go to his or her "center," and dispense with the power game of dominance. Your calm refusal to react also shows respect for the other person because you show no demand or intention for that person to change or conform to your desires.
Thus that person, assuming hopefully that there exists a deeper and appropriate connection of love between you, is blessed by the opportunity to cease co-dependent behavior also. Where that connection doesn’t exist, you might at least win his respect.
In the end you both will be happier. Of course it is possible that your effort to remain centered will provoke further abuse. If you can absent yourself from such circumstances, that may be necessary. If you cannot, you must either take more aggressive action to stand your ground (as calmly as you are capable) or endure what must "come of its own," with equanimity and calm acceptance, placing your faith in the Supreme Spirit.
It is true that a more spiritually advanced soul can sometimes take hatred and violence without a ruffle and thus even convert his self-styled enemies by the power of love. But most people struggling as you described are not as likely to benefit by starting there. Better to begin to untangle the knots of co-dependency right where they are by bringing under control not another person but your own habit-born reaction.
This is a complex subject and I suggest you might seek an experienced and wisdom-guided counselor, especially one with a spiritual perspective.