Write it out.  Some use a journal, write a letter then burn it or write it out here.  You do have to get it out or it will eat you alive as they say.  It might morph into self gathered as in why did I let that happen to myself.  If you can afford therapy that's great too.   

While DH and I are still together and are finally on the "same page," we had a time when it was not like that. It took 12 years and a humiliating event for me to say, "I'm done." DH and I attended counseling, he made some major changes and our marriage is still intact. I have not seen his kids in near 5 years. 

It took me near 4 years to really purge all the negative emotions. I had a great counselor (I did personal counseling in addition to marriage counseling). I did a lot of reading and group work (a catalyst for me was Forgiving What You Can't Forget by Lysa TerKeruerst (it is Christian based but I feel anyone could gain insight from the book and it's principles). I also read a lot of Brene Brown (secular) stuff. She's an awesome Scientist of emotions. I learned to set healthy boundaries. And I finally got to the place where I realized my disdain for DHs kids was negatively affecting me way more than they were being affected. I learned to embrace my "win" so to speak. They are not allowed in our marital home (boundary set in counseling) and DH sees them away from our home, which is a huge inconvenience for all of them.

In your position, you've free'd yourself from the entire situation. Realize they cannot harm you anymore, that you did everything you could to make relationships work and that those people are just toxic and life will eventually catch up with them. And even if it doesn't, you are FREE from anymore hurt or pain from them. Get yourself a good counselor to help you process these emotions in a healthy way and thank God (or whomever your higher power is) that you got the hell out of that mess.

Best to you. Life does get better. I know it's hard to see that now in the darkness of your anger and hurt but it does get better. 

I think it just takes time.  I kicked Killjoy out three years ago, just got my hubby back this summer.  Anger still triggers now and then, nothing like it was before hubby came home.

Flare ups can include intense rage and verbal abuse.  Then I feel ashamed of myself.  But they are getting far less frequent.

If you figure it out, let me know.

I think there is something to becoming obsessed/addicted to the chaos and drama that step-life brings. It may draw out a primative fight or flight response in our brains because our other half, our life partner, is so closely tied to all this other stuff. Stuff that we had no say in, have no control over and often can't change. The step dynamic creates it's own "dust devil" so to speak that we get swept up in.

Add to that whatever your own childhood hurts, tragedies or life struggles bring and it can become a complex cycle that may not be positive but it feels comfortable or familiar in some way. Even if it's miserable it's still busy and keeps you on high alert. I think the stress hormones, although extremely unpleasant, give a type of "high" that can be hard to just let go of. For those of us who stay in the relationship the step-family problem is never resolved. Personally I find it very hard to let go of things that are not resolved but I am getting better at it. I don't know for sure but think all of this is very specific to how women/step-mom's deal with emotions.

How to "let it go?" I don't know. I think it helps to find the things that bring you joy and peace. Try to rewrite those pathways in your brain. If you can, do a pilgrimage, go skydiving, try medical mushrooms or something extreme that takes over and jolts your brain into a new mode. I find reading, gardening, art, and exercise to also be good for getting my mind off of repetitive, useless step-sh*t and all the other crap that life throws at me. It's easy to turn to unhealthy habits as temporary fixes but that isn't great in the longterm. I keep searching for how to reconcile everything that has happened and everything that is, accept, be grateful and not waste my life on regrets and grudges.

to this day.  And I haven't seen hide nor hair of the three ferals since September of  2009!

You may feel like you lost in this situation, but by taking control and getting away from that toxic environment, you actually won. Those skids and your ex BF have to live with themselves, and other of their kind - in that toxic stew they have brewed up. That is not something I would want to live in/with. You are the winner here. Take the win and be happy. That is the best revenge. Therapy is also a great way to get out your feelings and sort through them in a healthy way.

I have recently went through this same thing.  I left at the end of May.  Divorce was finalized about 2 months ago.  I received one email from him, sugary sweet and wanting to try to "make things work" and some sh&t about his son wanting to go to college, blahblahblah.  Of course, it was ignored, forwarded to my attorney with my ex copied in the CC part. 

I completely understand your anger.  I have it still.  Granted, I am so much happier being away from those worthless pieces of sh&t.  But, I have anger.  Anger at myself for putting up with that, for not standing up to them like I should have, for devoting years of myself and my time to a hopeless, thankless situation.  I relive moments of humiliation they both put me through at different times and it's like I'm there again, in my old home, being embarrassed and verbally degraded by the very man who was supposed to love me - and his son smirking at me in the background.  It's agonizing.  I have been in counseling which I find very helpful.  A month ago or so I really felt depressed, I went to work but that was about it.. I stopped talking to my friends, barely talked to my family.  I've never been like that before in my life.  I wasn't angry at any of them, I just felt so down and depleted.  My counselor said it was grieving and that sometimes it happens later rather than sooner.  I am not on meds as I realize my feelings are situational and I am processing them.  But wow.  This anger and the accompanying intermittent despair.  I have never felt anything like it.  have to make a conscious effort to NOT be rude to my patients or others.  The anger is overwhelming. 

I would recommend counseling and journaling also.  Journaling really helps me.  Another thing that helps is doing nice things for myself.  Treating myself nicely since I was abused for so many years, like getting a massage, a mani/pedi or ordering out from a nice restaurant and watching my favorite movies that I wasn't allowed to watch when I was married.  I'm feeling better now, though my counselor told me this isn't just "okay, I feel better now, moving on."  It's not that you're just "over it" one day.  It takes time to process, to work through things.  With the anger come sadness for me, for my ex and his son.... no, I don't want them back.  But the whole situation was just ... unnecessary and avoidable except that my ex never recognized he was the problem between his abusive treatment of me and the codependent, enmeshed relationship with his son. I cringe to think about them and their future... or lack of one.

I also find comfort in my nieces and nephews, from age 21 to 5 and even my ex-stepdaughter, now 18.  They all love me, unconditionally.  My oldest niece and my ex-stepdaughter are like the daughters I never had.  Their love helps my heart, it makes me feel whole.  It helps me to see that I'm not this horrible, selfish, rotten person that my husband and his son made me out to be. 

Get close to your family and loved ones, hold them tight and hang on. 

Please take care of yourself, it will get better.  The hardest part is leaving and the immediate aftermath.  Chin up, move forward... we're all rooting for you!