Loner is sort of a derogatory, or negative term. Introvert is better and as someone familiar with Jung's view on introversion and extroversion to some degree can say, I'd fit the bill.
I have no problems socializing, it is just that doing so is physically exhausting for me, the social gathering I've had to spend most effort in, AKA I got a lot of attention lasted about 30 minutes to an hour or so, then I went outside to relax for a bit and ended up falling asleep for a good 10 - 30 minutes only to be found and shook out of my sleep. Being an introvert usually brands me cold, anti-social, shy, and more. But the simple fact is, being alone with my thoughts, a book or some other entertainment is usually more relaxing, fun and generally better than being with other people.