It makes me so sad when I hear that people want to destroy or banish a part of themselves (even though I, too, go through this feeling more often than I'd like to admit). Keeping in mind that I am brand-spanking-new to witchcraft, and my views are from more of a mental-health perspective (which I'm feeling are actually more intertwined than I expected).... Here are my 2 cents.
You mentioned that you have done a little shadow work, that you know (or have a pretty good idea of) the causes, background, and triggers of this behavior, and you have acknowledged that this behavior has served you well in the past. This last part is, to me, arguably one of the most important parts.
I wonder if, instead of fighting the part/behavior/what-have-you, and trying to get it to go away and leave you alone... Perhaps when you see it come up in your life you can thank it. Let it know that you appreciate what it has done to help you in the past. Let it know that it is important to you, and that it has done a good job of protecting you. But also let it know, gently, that you have grown up and moved on from whatever threat it was protecting you from. You are safe now, so this part can relax and use its energies to support you in other ways.
I never quite know how this type of explanation is going to go over when I share it with new folks, so I hope I don't step on any toes or anything. I just hope it helps