Let them miss you

I (29f) feel like the relationship I have with (29m) has been different for the last few weeks. It seems as though his interest level has declined and it’s honestly such a sad, hurtful thing to experience. We’ve known each other for about 9 months and have been dating. I’ve read some articles that says when someone pulls back you need to let them miss you. I’m so used to seeing him often, talking everyday. And now it’s just different. I saw him once this week and heard his voice once. All contact is usually me initiating and when I do he acts normal. But he doesn’t initiate any contact with me. I don’t know how to cope with it, it’s on my mind like 24/7. Anyways you’ve found to be helpful when experiencing this? Any advice appreciated. :(