Feeling alone after boyfriend leaves

I’ve been experiencing something that happens quite often whenever my boyfriend leaves. We don’t live together but he stays over the weekend and as soon as he leaves I start feeling alone, bored or slightly sad. I start thinking I wish he was here and I want to see him again, then I start over eating, browsing social media, and experience a lot of anxiety. I just can’t get things done because of this, and the feeling lasts a couple of days. Could you please help me process this?