I can't stop thinking about something.
Are you sure? How long has it been on your mind?
I have been thinking about it from a long time.
It sounds like these thoughts have become a little obsessive. Don't let you mind be consumed by this.
I think about it all the time, I can't do anything else, I can't stop thinking about it.
I think it's a good idea then to try to get help about this, because it's not healthy to always think of the same thing. Wouldn't it be nice to finally be able to take charge of your own thoughts?