How to explain quitting your last job, and if it's even worth mentioning?

Depends on how long you were there.
I was at my last job for 12 years and didn’t leave on good terms, but I got along with my coworkers and the maintenance supervisor very well and used them as references instead of my boss,and my new employer was happy with that.
So hopefully there are people from your old job that wouldn’t mind being a reference for you.
but if not and you were there for just a short time I maybe would just leave it out.

I don’t know if this will help, but I have found that people who complain at work are usually the ones who care about the business.

“My last job was not a good fit.”

I wasn’t challenged.

I realized there were no opportunities for advancement.

It sounds like you left suddenly with no new job in place. You can be truthful and still be diplomatic. If they ask why you left, find something positive and true to say as a lead-in. Like, “I enjoyed (the company culture, the challenge, some aspect of the job), but unfortunately I had to leave to deal with a personal issue. Now that the issue is taken care of, I’m looking for a more challenging position in my field,” or whatever.

It’s like the sandwich rule for critique. Say one positive thing, the uncomfortable negative thing you have to say (leaving your job) and then one more positive thing. They usually won’t press you further about “a personal issue.” (Even if that was a personal issue from within the company.)

I parted with my last job because I felt it was time to concentrate 100% on making plans for my future by looking for employment that would be challenging, allow me to grow and fit my future needs.