Kind of a peace - making post - love letter to ourselves.
You could do that, and like you say it would be speculation, and as an alternative, you could write yourself a love letter, one that wouldn't be speculation, it would be real, and part of you taking your power back and insisting on loving yourself again. And you could make sure you include all of the reasons you love yourself, all the awesome traits you have, why anyone, at least those without personality disorders, would absolutely love to have you in their lives, including yourself.
And you could write another letter to your ex, but not send it, and beyond the anger and beyond the hurt, to the compassion for them and the tough road they walk, and the gratitude for the gifts you've taken from your time with them and the lessons you've learned.
Heck, you could write all three, with a pen and paper, we connect emotionally better with things handwritten than typed, it can definitely be restorative and cathartic, and you get out of it what you put into it. We look forward to updates!