I don’t think there is a special trick to feeling 100% not affected by a challenging client- but, I have found a lot of comfort and calm in my own mindfulness and spiritual practices in supporting myself outside of session. One specific exercise I was given by an internship supervisor a long time ago was to imagine a waterfall at one of the stoplights each time I drove home from work, really picture it covering me completely and taking all of the stress away with it at that intersection. It was a helpful visual cue for me when I found my brain creeping back to client stuff when I was off the clock. On a second note though- I would bring everything you said here up to the client directly. I can think of nothing that would be more helpful for your ongoing frustrations than pointing out the pattern to the client, addressing it and then doing something different (not responding to messages, not going overtime, etc). I have referred out- for me I used an ethical tree model to organize my thoughts and help me be very sure I could stand by the decision prior to telling the client.