Apparently, six years ago and using a different account (that I don't want to link to here), I left somebody a comment encouraging them to leave an abusive relationship. I didn't do the whole, "Dump them, get a lawyer, delete Facebook, hit the gym!" thing.
It was more like just contrasting their relationship with the healthy relationship they could have in the future and explaining the kind of things that needed to happen in order to create that future. I also reassured them about some things that worried them. If nobody told me, I wouldn't even have realized I'd written that comment - I don't remember having written it.
But write it I did. This morning I saw an email notification in response to the forgotten comment on that other account. It was the person I'd given advice to! They told me that my comment had motivated them to leave their abusive relationship, stay single for a while, go to long-term therapy, and learn how healthy relationships work. They also said they're in a mutually respectful and satisfying relationship now even though it was once so hard for them to imagine that a healthy relationship could even be real. They'd even moved far away from their abusive ex and started a new career that their ex had told them they were too stupid for.
It made me cry happy tears. It's so touching, and it's amazing to think I changed somebody's life through a Reddit comment - or, to be more realistic, helped them realize THEY were ready to change their life. I'm going to make sure to think about this the next time I'm feeling useless.