Hi OP. This time last year I had the manager from hell - I have never felt so low in my entire life. I posted about it on here and got some brilliant support. I wasn't sleeping, wasn't eating, I was covered in sores (stress) and I was throwing up most days before work.
I know it seems so scary, but the best thing I did was go to HR. I spoke honestly about what had been happening and what I'd been feeling - the employment issues section is brilliant for advice about how to do this. I kept it in my head that I wasn't going to let her win, and that I wasn't going to just leave and let her do this to someone else instead. I've since found out that one of my predecessors was actually signed off for 8 months in the end with work related stress related to my manager, so it had been going on for years.
I was applying for other jobs at the time, but my confidence was absolutely on the floor and I couldn't think of anything worse than having to sell myself at an interview. I actually cancelled one on the day because I couldn't think of a single good thing to say about myself!
Getting support from HR and my other colleagues was daunting, but it was only after I spoke out that things began to change. It was a weight off my shoulders.
It took time, and I did have to be brave sometimes - deep breaths and phone calls to my mum and DP outside the office before going in! - but I'm so proud of how far I've come. My manager has since been moved on, and I've absorbed some of her role. I'm ahead on targets, I'm been trained to manage someone else, and I love my job. I'm so glad I didn't let her take that from me. She will never do this to anyone else now and that's a great feeling.
HR will have seen it all before. You can just have an informal chat at first and go from there. I really hope things get better soon. Tomorrow is a new day and you can do this.