I'm not on meds or in treatment of any kind (although I'm supposed to be on lithium.) It takes a lot of hard work to stay stable on my own. I've had a long time to practice.
The main thing is learning when things are starting to brew. I try to stay on top of tracking my moods and triggers. Keeping very knowledgable about myself is key. It's not always easy since I also get into a big lack of insight. Sometimes I think I'm doing a great job and then when it's over, I look back and see a trail of disasters I caused...
Another thing that helps is I do have my husband who will point out when I'm starting to be an issue. He tends to be a blunt person so he's not shy about saying if I'm acting up.
I have lithium but I forget to take it (and then I lost my bottle.) I am not anit-med or for-med. I think it's a personal thing. I know the lithium helps me but I'm terrible at remembering.
As for therapy, it can be a huge help. The right therapist is a blessing. I had a really good psychologist until she retired.

I miss her a lot.