Ben, there are some very good things in this thread to remind you of your value and your identity in the Kingdom. I'm going to talk about something other than prayer and meditation - not that those aren't good things to do, because they are, but there are other things you can do in addition.
Go back as far as you can in your memory and identify the first time you felt hurt, sad, betrayed, like you were not good enough, couldn't measure up, didn't matter, left out, rejected. Think about how those experiences and feelings contributed to the core belief that you should be insecure in yourself. When you go back in your memory, pray and ask Jesus where he was during that time. I'll bet he will answer you, that he was there with you, and he knows why you have become so insecure.
Try to nail down the exact core belief. Is it that you feel unworthy? Unworthy of what? Is it that you've been told you're never going to measure up? Measure up to what? Somebody else? Maybe measure up to perfection? Now, when you have a specific time when you feel insecure, try to figure out what the immediate thought was right before the feeling of insecurity. What was the thought? Once you have identified the thought, take it captive and make it obedient to Jesus - that is, combat the thought that leads to insecurity, and remind yourself of your identity in Jesus, who you are as a dearly, beloved child of God Almighty and co-heir with Jesus in his Kingdom. The key is taking the thought and making it powerless by bringing in the power of the Word. Every time you feel insecure, identify the thought, and turn that thought upside-down, making your thoughts align with God's thoughts of you.
This won't happen overnight. It might take a lifetime. You know the truth in your head, but you internalize it with practice so that you won't just know it, but you will believe it.