It is totally possible to become a secure person after being insecure. I did it. But it is very hard to change your personalty and you will back slide from time to time.
There is some truth in 'fake it until you make it'. If you present yourself as a friendly outgoing person, eventually people will believe you. And then it is a LOT easier to be one.
This link is to a post where I talked about How I changed my personality
Warm regards, Rick