If it were me, I would simply say I took a personal sabbatical, which is in essense what you did - you intentionally took a break from working. I'd even put it on my resume as-such, and leave it at that.
If they want details about what you did during that time, you can give a vague overview, highlighting any hobbies or travel that you may have engaged in, but overall sabbitcals are fairly understood to be time where one is attending to personal interests that are not work related.
While your therapy could indeed (and hopefully does) have a positive impact on your ability to work more effectively in the future, this detail is far beyond any one else's business and I don't think you should feel compelled to discuss that. I also don't think there is any positive outcome to discussing it. I personally would refrain from bringing up that aspect of your time off.