Why do I get depressed every time he leaves?

Every time my now ex boyfriend left me after staying over at my house over night, I got all depressed, for a full day or two. I'm 24 and this was my first proper relationship, is it normal to feel really down after your boyfriend leaves. We were together for a year, split up 3 months ago and just last week meet up, which led to him staying over, in the morning I felt really depressed, just like I used to get. Do you think the fact that I'm feeling depressed after seeing him means that it's him, and that he's upsetting me, or is it just me, maybe I'm not ready for a relationship. All help would be really appreciated thanks