Thanks for the thoughtful replies! I have not had a career in academia in the forefront of my ambitions before starting the PhD. I always thought I would take the things I learned from my degree into the workforce straight away. I applied for the PhD just in case it turned out to be the right choice, and in case I couldn't find anything else straight away. When I had a hard time finding work, and the PhD was applied for and ready to go, I kind of slid into it. My friends and family want this for me, so I definitely don't feel like I'm letting them down. I have been questioning my decision from the start, and can see myself doing so in the future. However I've come this far feeling this way, maybe I can just keep on pushing.
I take your point Mackem about accepting the burdens and the sacrifices that come with the role. I had previously attributed my negative experience to the fact that things change as we progress through various stages of our lives, and I was lamenting the way things were in an earlier stage. I was made to feel worse by comparing my experience to friends who followed different careers being less burdened, and other students being successful. But you're right, the PhD is isolating, and requires so much time that can have these effects.

Here's my case.
I'm currently 1yr into my PhD. My project has many qualities that I thought would excite me about a PhD before I started; great long-term fieldwork, conservation focus etc. Though I have been feeling queasy about it since fairly early on in the piece.
Before I started, I had just done the honours project of my dreams (like 1yr Masters), and had a relatively smooth run with two good supervisors and excellent fieldwork - understandably I would feel comfortable choosing to stay with one of them for my PhD though on a completely different style of project.
There was one alternative PhD option at the time; which followed quite closely from the 'dream' honours project and could have suited me well. However I felt I had done the dream job already, and would find it less stimulating to study the same system for my PhD. Perhaps more influential however, was that I feared the responsibility of running that kind of project myself, and what I would think of myself if I failed.
Hence I applied for my current project, thinking that it would overall be a less scary one. Though prior to starting I had some serious changes of heart, other career choices long thought about yet never acted upon that started tugging at my heartstrings.
All those I spoke to (friends, family) were charmed by the lustre of the 'Dr' title and didn't understand why I could want to do something different, seeing all other options as lazy by comparison. Furthermore, my project is quite a sexy one in the eyes of many of my student and staff colleagues - so I find it hard to find anyone that sympathizes with my desire to leave. Even furthermore, I'm scared that I'll regret my decision to leave once I do, even if I manage to get on another career track.
At the moment I feel like I have very low self esteem, and lack the confidence to make a choice so I'll probably drift on through. However I'm worried that my self esteem won't improve and as a result many other areas of my life will go neglected. So I've read a lot of posts like mine already and thought I'd feel empowered by writing my own.