Where rumination gets dangerous, in my experience, is when it becomes an infinite mental loop.
Whenever people talk about moving on, I feel as if they were talking about using their third arm, that I just don't have.
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.
The question then becomes, how do I make positive experiences? If I make what I think is a positive experience, I end up worrying about parts of it and wondering about its negative aspects. That's also something I have to work on and change. It's a cycle, it's slow and iterative, and I feel like I'm making progress, however little, but I am how I am right now, and I'm okay with that.
A good project is your best friend
Call it "escapism", except without the feeling of worthlessness that comes with drowning yourself in fiction.
It's self explanatory. I've used it every day for a few years now. It makes me feel far more centred, far less anxious, far more able to deal with the swells and dips of daily life. It has also made me face those fears about parents or myself becoming sick as an inevitability that can have a beauty to it. Ignoring it won't make it go away or any better, just all the more painful when it railroads into your life.
Wish you well my friend, good luck with whatever you do, but if this approach feels overwhelming, definitely consider having a chat with somebody about it.