Time to let go (or at least try!)

Ruben says that I have the tendency to dwell on the past too much, and he's right.
I can't help it that even after a few years I still have so much heart ache over a friendship that I put so much into, gave so much for, and after several years of being friends our friendship was turned into a nothing more than that of mere acquaintances - priorities changed, and I just wasn't one of them.
Or how about a case of acquaintances. A friend of a friend. Someone I knew and considered a friend, and tried to set up with a very good friend of mine, but found that his friend was actually a better fit for her. When I learned that she lost her job and was looking for another, I stuck my neck out on the line - as well as my job, so that she could gain employment. Later I found that she twisted words, made up stories to ruin my relationship with other office friends, stabbed me in the back, then basically copied my resume, interviewed for the same positions as I did, and talked badly about me during interviews. My heart may not ache over this one, but it surely taught me to never put my reputation on the line for others. But what does upset about this "friend" was all the other friends she cost me, not to mention the fact that she nearly cost me my current position, as well as the one I was later hired for. I still remember that she had everyone going, saying that the only reason I had my job was because of my husband. Sorry but my resume spoke for itself.
And let's not forget the bitch at work who went around spreading rumors based off of nothing, about me & my hubby's sex life. I always hated the fat bitch cause it seemed like she was hitting on my husband telling her details about her life, while he was her boss. But after the crap she pulled, I really hated the cat-piss smelling, ugly, fat bitch. Luckily she quit contacting my husband for non-work related items when he started ignoring her emails and phone calls. Even still though, I think I would take great pleasure in kicking her ass.
So how do I let go of this crap once and for all? I can't help the fact that I am a very emotional being - and that I can easily hold a grudge and never let go. I mean after all, these items here are just a few of the many scars I carry. I've tried to re-build lost friendships with a few, but they never really took off the ground. My guess is because it's an issue of trust now. How do you get past that and learn to trust again? How do you put your guard down? Especially when the one who twisted my words, lied, made up stories, and stabbed me in the back is still in the picture - they wonder who to believe, the person who seems like a friend because they have never hurt them, or the one who walked away hurt and angry, but did so without a fight?
What's really kind of funny to me is that chemo seemed to wipe away most of my memory. I only have few memories of being young, growing up, friends in high school, and names.....I can't seem to remember any names beyond the last decade. Yet through all this mind-wipe, I still remember a whole lots bad stuff and hardly any good stuff. Sucks, huh? Maybe one day I'll be lucky enough to forget all this too, and my scars will heal.