I was incredibly insecure in my first relationship. (Probably about a 10 back then.) I know that level of insecurity put a lot of strain and pressure on the relationship, causing events that led to its end. I think insecurity comes from looking to the external world to tell you who are you and what your value is. When you are this insecure (in my experience) people are like mirrors, and you work to see yourself reflected back in the ways they treat you. You create problems in relationships this way.
It took awhile for me to figure out that no one else determines anyone elses' worth, that your existence makes you as entitled to everything the world has to offer as anyone else, not your abilities or merits or looks etc. You don't have to accept being insecure. It may sound cliche, but the beginning of insecurity's end is the beginning of really getting to know yourself and befriending yourself. I agree with Zencat, that when you develop a stronger sense of self-esteem it makes you secure in your relationships. I think it's because you don't need anyone to place value on you to feel valuable. This way, you are able to truly enjoy the other person and accept them and form a much deeper bond than if you were constantly seeking fault in them as it is interpreted to be fault in yourself.
Now, I'd rate myself at 2.