I do think some dreams have important meanings, particularly if they're recurring, but I don't think this one had much of a reason behind it. If this were a common occurrence I'd probably ask if you were worried about anything in your relationship with your girlfriend (though if you've been going for almost 5 years it can't be all that bad!), seeing as it revolves around her quite a bit. To be honest the whole thing, to a psychiatrist, would scream 'are you doubting your relationship' with the parts where you go to your room alone and don't like it, give each other a final goodbye, and when you're talking to her at the end but you're not together as your dead... Such a sad story, I've gotta say.

Personally I think it was just your mind wandering and I wouldn't worry about it, unless it comes back obviously. Just use it to see how grateful you are that your happy and healthy.

Hope this helps.