I used to hear "just let it go" and think I was being asked to do something impossible! Using a third arm is a great way of putting it.
Turns out the self-regulation needed to "move on" is just a developmental skill. Like any skill, it can be practiced and improved over time. Hope you find this encouraging!
> Can you stop thinking about something only by wanting to?
Absolutely, yes. If I catch myself in a death spiral of thoughts, I can literally tell myself "no, that's enough, I'm done with this" and do something else instead. It takes introspection, acceptance, understanding, compassion (for yourself and the subjects of your thoughts), therapy, and practice. Meditation can also help.
There's no single piece of advice I can give because it was a lengthy and painful process, but maybe it's enough to say that yes, it's possible to learn, because it's something I had to learn and I managed to figure it out.
It sounds so cheesy to say, but I really wish they would teach mindfulness in schools. It's such an important life skill, and it's one I had to realize on my own was important.
Whenever people talk about moving on, I feel as if they were talking about using their third arm, that I just don't have.