Grandparents trying to pretend that I don't live in the UK

I love my grandparents and even though I have been telling them since last September that I am going to extend my stay in the UK they insist on believing that I am coming home for good in April. Every time I call they ask when I am coming home - I wish there was a way I could be more firm with them, but I don't want to be mean.

Has anyone had a similar problem? And how did you deal with it? I don't know how much longer I can take this guilt trip! 

Yeah I have the problem now. I know how it goes. I just always change the subject and tend to ignore it. I realize they only really miss me and that's why they do it. I feel really guilty every single time. But it is my life and I get to decide where I live so I remind myself that. Also, we're adults. Even if we lived in the US, we'd still be living on our own most likely and would only visit our parents/grandparents during holidays anyways. It's not like we'll never seem our family again.