Hi Sarah,
I've been in a similar situation in the past. I didn't make the best decision (added computing to my original degree). We can't always see into the future however carefully we weigh up the options, but everything worked out ok for me in the end.

So I would recommend that firstly - you are the only person who can weigh up your motivation and your state of mental health for the future. Studying is hard. Teaching high school is more than super hard....

Personally I would not recommend an older adult going into high school teaching. It is a burn-out profession and you need to be temperamentally suited and to absolutely adore the job to make it through to retirement age. I have many friends who went into high school teaching because it's a secure profession and some ended up hating the job for the same reasons that you mention = changing state policies, controlled curriculum not always suited to the students. Many got burned out by age 45 and had to grit their teeth to make it through to retirement - with depression, anxiety, insomnia. Only the ones who adore spending time with noisy young children no matter what government policy stay relaxed the whole way through.

I would seriously consider doing what you love and going for the specialist Masters. Yes, it's a financial commitment and might not pan out for you BUT I get the feeling that it's a serious option for you. Through dealing with your issues, you have acquired an aptitude for connecting well with a range of people, so use it !!!

One way to decide would be to apply and study the Mastaers curriculum of a number of universities in some detail before committing yourself. Listen to your reactions inside. I found myself taking my degree in a completely different subject than I first intended because at the last minute the curriculum for psychology (which I intended to study) seemed totally yawn-inducing to me. It was the right decision. Psychology interests me in general but not as a profession, and not the way that it's studied in university.

Good luck to you and way to go!

As an aside, would teaching multi-ethnic literature in the prison system be a possible career? You know how much I admire your work with women in prisons, and creative writing is surely something that prisoners can benefit from doing. Creative writing in general might be a way forward for you.

Saidso