If you’ve felt insecure in a relationship, what could your partner have said or done to help?

Well it depends on the root of the insecurity. For a general (unfounded) sense of insecurity a partner who says they love you (often, meaning it), show they love you (small romantic gestures) and show (say) that they find you physically attractive can should bring some security.

Only in one I felt that way and he could have changed his actions/behavior.

If you feel insecure, that's not your partner's fault. It's yours

Look bish...get your shit together before i give you a real reason to be insecure.