What are you insecure about, if I may ask? The first step (which you have already done) is realizing that this is a problem. The next step is figuring out why you have this issue. Only then, will you be able to start to improve
things.
Also, other people can contribute to the problem as well. For example, you are insecure in your romatic relationships. If your partner is flirting with, or hanging out with other people of the opposite sex(more than friends), this might be a problem they are causing, rather than just you being insecure. Do you understand?