I'm sorry that you are so unhappy in your present job.
Last July I left my teaching job and decided to do supply. I was very worried about whether I would get any work, but I really wasn't happy where I was and I decided to take the risk.
I suggested to the school I was working in that I might be available for supply and they were really keen! This surprised me as I hadn't been that happy there. But they knew I was a decent teacher, and I knew the children well, and by employing me directly they saved money on a supply agency. As a bonus for me, I didn't have to give an agency a cut either and I still get to pay into my pension (yes, I'm old enough to worry about that!).
Since then they have offered me fairly regular work. Encouraged by this I got in touch with 2 schools where I worked previously, some years ago. Both schools have also offered me supply work as a result.
Combining all three schools, I now work an average of 3-4 days a week, which suits me fine, and it is so much less stressful than being a class teacher (in schools where I know the children and culture, at least). I have had some very lean months (December!), but I just accept this and use the time to pursue other interests and do my housework! Other months can be manic, with everyone wanting me on the same days, and having to turn down work.
I think you have to treat it seriously, like a 'proper job', and accept that schools will never quite treat you as a member of staff (I miss the sense that I know what is going on behind the scenes). But overall I love it, and am seriously considering not looking for a permanent job.
I know many people will say that schools are not using supply teachers any more, but that has not been my experience so far. I would suggest sounding out any schools you know BEFORE trying an agency, as you are better of working for them directly.
Good luck.