Why can't I let this go?

I've been without my computer for almost a week so I haven't been able to update, and trust me I felt like I was going to die.

Anyways, as you all know my dad has been staying with me for a week now and it's been great. We've been applying for jobs and fixing his resume, we've been watching movies and cooking together; it's been great. Today he got a call from a friend 20 minutes from my hometown and said he has work for him to do tomorrow where he will be making $10 an hour. He needs the money and definitely needs the work, so of course he took this guy up on his offer.

He told me about it and I said that I don't like the mind set he's in when he's in my hometown, and I don't like that he's going to be that close to it.  He looked at me, smiled and said, "Don't sugar coat it baby." I looked away and tried to act like I was happy he had work, but I just couldn't do it. It's temporary work that will pay his bills for another month, and I should feel happy for him, and it's not like he won't be back here where I live next week. I don't know why this stressed me out so much, I just can't get it out of my head that he will relapse. I can't get it out of my head that he will end up disappearing and never come back.

I helped him pack his stuff, and got all of his paperwork together so he could study for a test he has coming up soon. I helped him get everything in his car, and gave him a big bear hug and he said "I'll see you on Sunday for father's day." I nodded and watched him get in the car and drive off.

Why can't I just trust him and let him do his own thing without feeling this painful hand on my heart threatening to rip it out of my chest? Why can't I let what he has done go? Why can't I just truly forgive and forget everything that has happened in the past? I want to so badly. And what kills me the most is dad knows that I don't trust him, and he knows that everything he does, I have to look at logically instead of with trust. I see it in his eyes that it kills him that I try so hard to trust him, and can't, and he knows there's a damn good reason for it. He knows it will never change, because in all honesty, who can get past something like watching their father battle a crack addiction? Who?! Definitely not me.