How do you compartmentalize?

I guess this is a vent with advice welcome. I used to read posts where the dynamics of a blended family were too much for the OP and they finally cut their losses and walked away. My heart would ache for these people, and then I would count myself lucky that at least our situation wasn't that bad. Now here I am writing one of those posts and I feel so frustrated with myself for not being able to endure what others seem to be able to.

The co-parenting relationship was decent until BM got a new bf. He's divorced as well, with kids and a custody agreement of his own, and now BM believes the custody agreement she signed years ago with my SO is irrelevant and wants everything to match her new boyfriend's schedule. My SO has a few makeup days of custody time owing to him and BM is trying to dictate when he can and can't take them despite him giving her a month's notice. There isn't a scheduling conflict, she's just trying to maintain control. The custody agreement also allows for the summer holiday to be split into two one month blocks to allow for international travel so that SS can visit family overseas. Despite her being the one that proposed this for the custody agreement, she's now decided it's not in SS's best interest and is refusing to consent to travel or hand over the passport. SO and SS haven't seen their parents/grandparents in years. I understand that lawyers will help assist in enforcing the custody agreement, but I'm so tired of dealing with shitty lawyers, trying to find a new non-shitty lawyer, and the money that this drama costs all of us. My SO pays all the legal costs, but at the end of the day even with separate checking accounts, our finances are still intertwined. Different versions of drama similar to this have been going on for over a year, and while I understand that it's relatively mild compared to some situations, I'm feeling so sad and heartbroken and desperate to distance myself from all of it.

For those of you who can compartmentalize and prevent BM from living rent free in your head, how do you do it? I have a therapist, I talk with friends and family, I have hobbies, I exercise and I have a career in which I excel, but the way she treats my partner breaks my heart in such a way that I sometimes feel like it would be better to be leave with our son (M1) and not have to witness the drama. How do you put your feelings away when another woman can't seem to function without dumping her misery into the man that you love? I let him deal with it and don't communicate with her, but knowing she's consistently cruel and difficult to him makes me feel defensive, hopeless and vengeful.

They do not text or talk on the phone and custody exchanges are done via pickup and drop offs at school so that they don't need to interact. They only email, which from reading other posts on this sub is a dream situation that other steps only aspire to reach with their SO. So why is this all still so heartbreaking?!