Let it go?

I find it so amazing, that others have the ability to let go of something so easily. Be it a friendship, a relationship, an environment, a hobby. How do some find it so easy to move on with their lives and pretend that what they once were so emotionally invested in, never happened in the first place? 

I want to be able to move on too. Honestly, it seems so much easier to just care a lot less. People seem happier, they don't get caught up in unnecessary thoughts that they struggle with or emotions that they must control, all they have to do is realise they need to let go and they do. They find other sources of joy and move away from what was being toxic in their lives. 

Yet, I find myself stuck here. As an attachment begins to cease, I inadvertently find myself thinking about the What ifs, the How could Is, the Maybes and whatever other stupid rhetorical questions that I could possibly ask myself. I go over all my actions in my mind again and again, wracking my brain to find out an answer as to how I could have ended up in such a situation. I usually end up with wanting to slap myself and scream and cry because I wonder how sometimes I could be so stupid to let all the unnecessary hurt come. My head and my heart remain rooted, refusing to move on. The thoughts won't end nor will the emotions. 

I want to let go so badly. I want to stop feeling so vulnerable and paranoid. Why is it that I care so much? 

Becoming so emotionally invested only seems to be a bane rather than a blessing. 

How do I move on from here? 

Is it so bad to even begin caring about things in the first place? 

"How long must I wrestle with my thoughts and everyday have sorrow over me....But I trust in your unfailing love, my heart rejoices in your salvation."