Hi PeachCream, maybe just habit?? If you were used to having him around and if some of the things you're thinking are things you'd automatically think/say if he was around...........
The only "worrying" bit is the "all the time" though, you're almost making it sound "obsessive" unless that's just the way I'm reading it??
I'd say that you need to focus on letting your thoughts not exclude other people from the parts of your life your relating to him. If you can do that then maybe other people will gradually replace those thoughts, and you'll be more saying what you're thinking to them, instead of thinking them in relation to him. If that makes sense??
He might then still be in your thoughts from time to time/now and then (that's natural) but that needn't mean that you haven't moved on if you're still able to "let others in" and don't allow your thoughts to push other people away/block them out.
Best wishes
Alison