Look, i'm in the same boat as you here. I haven't been with my boyfriend for as long as you with your girlfriend, or lived with him, but I have the option of Exeter which is very much in my comfort zone - 1.5hrs away from my parents/other family, I used to live in Exeter, I have friends around and family friends who could take me in in an emergency.
Or I can go to Southampton. Which seems like the perfect course for me.
As horrible as it is, people do say to prioritize your education. But I struggle to make new friends and get to know people (which I put down to anxiety :s).
But at the same time, if you're not going to be happy, don't do it. Not going to a prestigious uni isn't the end of the world! You can always (if it's in your bigger plans) transfer if you need to, or you can do a PhD at another university. It's not the end of the world if you make what you might feel in a year's time is the "wrong" choice.
I've always lived my life by going through pros and cons and what's logical, and I genuinely believe that I have some sort of natural intuition about where I should go and what I should do, I've had this with a lot of my academic choices so far, and none of them have turned out to be bad choices. If I was to go with this still, I'd be at Exeter.
Sorry if this is very rambling-y, I'm really bad at phrasing what I want to say. I just wish you good luck in whatever you choose, and please remember it's not too late to change your mind after you apply there. Sure it's a hassle, but not the end of the world.