NC, although this is still probably a bit outing. ExP broke up with me a few months back now after a long term relationship with a couple of struggles towards the end, they're avoiding contact around tying up loose ends (house ownership, belongings, etc). They didn't tell me why the sudden breakup, moved on immediately, gaining a reputation, lost all self-worth and respect, spreading some horrible lies about me, etc. I've desperately tried everything to learn what went wrong or to make it better, but ExP is content with their new life.
MIL has been in touch with me to learn what happened and deal with this amicably, and she's a lovely person trying to handle this nicely for the both of us.
I've been extremely upset by the breakup, maybe more so about how ExP has been behaving and the damaging lies that have been spread about me, coupled with countless lies told to me.
MIL met up with me to discuss things, and I told her some of what has been going on. Probably TMI, but I was hurt and still am hurt by what's happened, and I'm always honest with her. I've not said anything that is not true. She was mortified, despite me not giving even half of the details.
AIBU to have told MIL about ExP? I feel like I shouldn't have, but friends have said it's not a big deal given what ExP has done or said about me.