Insecurity is mainly about lacking self-confidence and not trusting YOUR own choices/decisions based upon bad past experiences. At some point you have to let "yesterday" go. Naturally your mate can (attempt to offer you assurances) but overcoming insecurity, building self-confidence, and trusting your own judgement is something only you can give yourself. It's not your partner's job to rescue you but rather it's your job to address your own issues and strive to be as complete a person as you can be for them. Personal growth is just that, personal.
You gain cofidence by accomplishing small goals, taking calculated risks personally and professionally. With each "victory" you begin to trust your instincts a little bit more. Everyone is insecure about something at one time or another in their life. To be human in a world of uncertainty is to have to deal with doubt now and then.
Life is about learning from our mistakes, not allowing them to imprison us!