Crying whenever my SO leaves

My boyfriend and I have been together for almost 6 months now. We started our relationship long distance. Every time he left after visiting for the weekend, I would cry. I'm not talking about a quick 10 minute tear fest either. I would be in full on hysterics. Honestly I thought it was the long distance and not being able to see him all the time, which was extremely difficult.

He left this afternoon to visit friends which is a four hour trip. He's staying there tonight so I won't be able to sleep next to him and I'm struggling with holding back the tears again. I don't understand why I'm like this. Its not like he's going to be gone for more than 24 hours. Does anyone else experience this?