When we experience extreme pain in relationships, we tend to make up stories that allow us to stay in the cocoon of the relationship. That way, we can avoid feeling like we are betraying ourselves.
For example, we may think things like, "I've never experienced such intense emotion with someone, so they must be my soul mate. There are always difficulties with your soul mate, right?" Or "If I let go of this relationship, I will never experience this depth of love with anyone again."
The truth is that the attachment and dependency you feel may be intensified because of the "myths" about your partner and/or the relationship that you've had to create unconsciously as a way of dealing with the difficulties. Ask yourself honestly if the "pros" of the relationship you think about are a way of justifying it, despite your pain.
In my coaching work with couples, I've seen firsthand the very destructive power of hanging onto relationships that aren't ultimately right for those involved. When you feel emotionally tied to someone who brings more pain than goodness into your life, you create a vicious cycle: attachment breeding fear of separation, which then fuels further attachment and codependency.
In other words, you can feel completely tethered to someone, dependent on them for almost everything in your life, even if they are totally not right for you. For many, this is the hardest fact to accept in the process of realizing the need to let someone go.