Practice!  I keep busy doing my own things really, I'm a busy person! I am satisfied with my life and my life with DH and concentrate on that. SURE there's always a SD somewhere in there, lately less about OSD, but worries about YSD do crop up (I do care about her even through everything and hope she is well and grows up into a smart independent and kind woman).  But I don't let myself ruminate on it at all.

Depends. 
 

If they keep doing dumb ish and bio parent is a complicit enabler to their unsavory behavior then complete disengagement or worse .... ending relationship and leaving/divorcing. 
 

Otherwise I remember the more emotional energy I expend thinking about their lack of home training and the obtuse Disneyland parents who raised them to be nerve grating crumbsnatchers .... the more irritated and resentful I become plus less time I spend focusing on myself doing things that make me feel good. 
 

I also never got heavily emotionally invested in stepkids because I preferred to be more like a mentor so the less invested I was the easier it was for me to ignore their maladjusted behavior 

I find that BM, GF, and our steplife situation used to take up daily space in my brain. I was so hurt by the extent BM was willing to go to keep SS from DH - the lies, the personal attacks, the manipulation, etc. I was pissed at the family court system for allowing that behavior and giving BM what she wanted, and I was sad that SS wasn't going to be a meaningful part of our lives on a regular basis. I was sad that the court's logic of "a child needs their Mother" was the sole reason to prevent SS from having a meaningful relationship with DH. I used to think about it all several times a day, replaying different moments, remembering words BM and GF said, thinking of ways it could have been different. I used to have panic related symptoms like sweats, shaking, racing thoughts, etc. as a trauma response to everything we went through.

Honestly, counseling and time have been the greatest healers. Now, I am by no means perfect in this arena. I just had a dream about BM, GF, and SS a few days ago that messed me up for a whole day. But I find these days to be less and less frequent. Now instead of several consuming minutes or hours per day, I think about it all every for a moment every few weeks, maybe even months. Going to counseling and learning that this wasn't mine or DH's fault was helpful. Learning to accept the situation for what it is was helpful. But I have healed most with time. Finding joy in DD, exploring new hobbies, and spending time with people I love help. I hope to find peace in the next 6 years when SS ages out and we can block BM permanently and her legal bond to DH is gone. 

They will make the same/ different mistakes as long as they live,  disengaging from all of it is the way to go.  If you keep leading them to water you will get tired.  

I had to stop caring about what SD13 thought of me and my son. I used to constantly worry about what she was "bad mouthing" me for, trying to defend myself, thinking of ways to bring it up to DH, etc... 

I came to peace with, you know what, she can hate me, not like to be around me, not want to accept our son, etc... but I won't let it affect my mood. And I SURELY won't let it affect my DS3. Fact of the matter is, myself and our DS3 will ALWAYS be around, so when she is older, she can choose to not have a relationship with us if she doesn't want to. And I am finally okay with that. 

I spent a lot of time out in the yard gardening when they were with us EVERY weekend. (BM was too lazy to care for her children even though on paper she had primary custody, we had them more.) I have my own children to care for so I also stayed very busy with my children. I began seeking out and staying active with my own hobbies and interests. I reached out to my own friends and family for support.

I ignored SD's bickering and whining and let DH deal with his diablas 100% of the time. He made their lunches, drove them to their activities and so on.

The main thing I did for SD's was prepare a "family dinner" and the only reason I did that was so that DH, my BK's and I could have a nice, nutritious, delicious meal. SD's would make snide, snotty comments about having dinner together or the food that I prepared. I ignored them and didn't change anything about how I prepared the meals. DH allowed them to be brats which was frustrating but not my job to fix.

I taught my children to make polite requests and be courteous when making their personal preferences known. I don't respond to demands. When SD snottily whined "I don't like tomatoes in my salad!" I made sure that salads always included tomatoes.

As the SD's have gone off to college and BM moved out of state (hallelujah) there are fewer reasons for me to get wrapped up in rumination. One thing that helped me is I ask myself "Does BM think about me more than I think about her?" If the answer is probably "No, BM doesn't think about me at all." then you are allowing BM too much space in your head.

BM is self-absorbed and selfish. I don't think she thinks about me or my children at all. She thinks only of herself and how her own children can serve her own needs. I want her to think about me far more than I think about her. This helps me to stop ruminating and instead enjoy knowing that I am taking up more space in her puny head than she is in mine. This means I can hardly think of her at all because she hardly thinks of me at all. Same goes for Sdiablas.

I give myself permission to indulge in thinking about how annoying, stupid, manipulative, rude etc BM and Sdiablas are when it is relevant, such as right before, during or right after SD's visit but I keep my thinking about them contained to short periods of time. I don't think about them on a day to day or moment to moment basis anymore. It takes practice.

I think it's healthy and fine to let off steam now and again such as I am doing right now. We have to process the insanity that we're dealing with.

They don't deserve constant space in your head tho. Read a book, play with your children, enjoy nature, exercise, go for a walk, grow a garden, watch the sunset and forget about them.

I ruminate on SS way too much. I don't want to do that because 1. anything that detracts from being in the moment can only lessen contentment and add to stress. 2. There is so much I could be devoting my thoughts to that are positive and productive, including my own BS. Yet, I find my mind ruminating on SS, often right before and after a visitation.

I read something that I think explains this. The article talked about how humans are cued to notice and dwell on threats exponentially more than beneficial signals in the environment. If we ignore a threat, it could mean grave danger or even death. So even if it's a false alarm, humans are quick to notice and react. There is no downside to reacting even to false alarms. You run away, and find out later that it was just the wind or something. So there is no downside to repeatedly reacting to threats even if merely perceived (much better than ignoring it which might get us killed).

Although SKs are hardly dangerous, and certainly not mortally so, I do think psychologically, humans are naturally condtioned to notice and dwell on those who may threaten our well-being. If we perceive someone at work as a threat, we might stay away, or strategize to diminish their influence. But when it's someone you're living with and who is beloved by the one you love, then it becomes quite fraught ('the call is coming from inside the house!'). All that to say that the more I can feel safe, the less compulsion I have to focus my attention on SS. More recently, taking more of the reins in organizing SS's visitation has helped. It is making me feel less helpless, and more in control of my environment.

One very negative result of runimating on SS is that I end up dwelling in a head space that I feel very uncomfortable with. I am typically pro-social, respected by my colleagues at work and church, cares for and is cared by my family and friends. But when I start to worry about SS, I become internally defensive which makes me more closed and reactive. I'm less focused on how to help and more focused on how to protect myself and strategize around manipulative behaviors, etc. I'm trying to be more forgiving of myself because it is natural to react badly when someone is hostile towards you. But I also want to, as much as I can, be the person that I want to be rather than the person who is merely reacting to hostility.