Why do I feel lonely and empty inside?

I am a 20 yr old university student starting my final year at uni. I live with some friends and have a terrific family but I feel empty and lonely all the time. I realise that I should be grateful for things I have (Western stuff, family and friends) so I write down things I appreciate in a list, but then the feeling doesn't last long and this lonely empty feeling comes back again and I start to miss my horrible ex boyfriend. I want to know what causes this lonely empty feeling and how do I get rid of it so I can stop thinking about the ex? Because I think missing him and the lonely empty feeling are connected , please help