So, I'm sure no one is surprised to hear that I am one of those people who likes to sit with my feelings, make friends with them. A significant appeal for me with mushing is the emotional connection/content, just like it is with video games, just like it is with television. If I'm not crying at some point, I'm not invested enough to really enjoy myself, and I have a problem with being able to play if I'm not wired to my PC's brain on some level. While I'm 'plugged in', part of my enjoyment is experiencing my character's emotional content. I wrote a private journal-y thing a few days ago and made myself BAWL with it. Nobody will ever see that, I just like crying I guess.
It becomes a problem for me when:
a- the emotions continue after time away (turning the game off doesn't make the feelings stop)
b- I start making my feelings other peoples' problem (people on the game, people who share meatspace with me)
Sometimes problems are OK; something falling through after a RL year of work, I'm GOING to be pretty inconsolable, whether it's a tabletop character or a mush plan. If I strip mush-context out, can I explain it to a non-gamer and they'll understand? This is OK. These emotions get dealt with just like the same emotions that are caused by other major disappointments or loss or whatever. Friends, self-care, distraction, whatever.
Sometimes it's just unhealthy, particularly when you're talking about anger or SPECIFICALLY, feeling personally wronged by the choices of other CHARACTERS (not players, even though yes, players are behind them) -- this stuff is awful, and it's bleed that needs to be addressed immediately. If self-coaching / reminders of what I'm dealing with doesn't make the feelings stop (talk myself down out of the tree, these people did not just kill my darling), then I need to unplug and walk away until I'm feeling like a reasonable human being again. This actually includes the feelings that happen in reaction to genuinely being wronged, to be clear -- in the moment, it can't be addressed in a healthy fashion, so you really just do need to unplug until your behavior is at least under control.
ETA: I mean, I am still fucking broken about (spoiler) in FFXIV. I played through that eleventy billion years ago, and my ass takes my PC to the in game grave with some regularity. I LIKE CRYING.