Bits of what you say are silly. Bits are not.

You are leaving somewhere, and a time in a place you've been happy is ending. These are natural things to be sad about.

BUT. You are not losing the people that have been special to you. You are not leaving forever. From the discussions we have had, it seems that your family and london home are not objectionable in themselves - they are simply not the same peak of fulfilment that oxford has given you.

libellum is right, you could stay in oxford the whole time. Even if this is too drastic, you are not far away, and there is nothing to stop you visting. Why not oscillate and spend a few weeks staying with friends here and then a few at home? Or, if work is needed, spend the week in london and the weekends in oxford. Or something of the kind.

I suppose what I'm getting at is: yes, leaving is sad. But it is not an absolute thing, it can be compromised and adapted to your needs.