Letting Go

I've never been good at emotionally letting go.  You know, moving on from something or someone where the situation is painful or harmful in enough ways that logic says to move on, yet the "good" or what I'm getting out of it is so strong that I fear losing it more than I fear the bad of the situation.  The darkness of that potential loss is worse than the reality of current pain.

What the heck is that about?  I don't know why that has such a hold on me - I try with every tool that I have in my sadly empty toolbox to remedy the situation: I ask friends, I journal, I logic it out - but nothing works.  At the end of the day, my emotions actually seem to beat my brain into submission without my permission or intent.