Did this just start all the sudden or has she always done this? Did you just change her bit type at all that could be causing discomfort? I agree with tinyliny on the training and also checking for any dental issues or mouth sores first.
If it does wind up being something she is doing just to get out of work I just wanna ask --when you put her bridle on, do you bring your arm up over her head and between her ears to grab the headstall --then guide the bit in with your left hand? Just curious. I use to do this, but my Arabian mare figured out she could just lift her head and I couldn't follow her because I'm shorter and once she raised her head to a certain point, my arm didn't reach. When we checked it out and saw that she didn't have any mouth issues, my friend showed me another way that makes it harder for the horse to get away from you when you put the bridle on. I now hold the entire headstall in my right hand using my thumb and pinky finger to kind of spread the sides apart where the bit is so that they can comfortably take the bit. I then use my left hand to guide the bit into her mouth. That way, if they do start raising their head, you can just follow them with the bit lightly
up against their lips. Don't try to get it in her mouth at first, just follow her so that she realizes wherever she goes it will be inevitable that she will have to take it (well, I guess this depends on the size of your horse!
). Also, I noticed my mare is more comfortable now that I don't put my arm up and over her head so she doesn't feel as trapped. Since I've been bridleing her this way, she has NO problem taking the bit because she knows she can't escape.
I've tried pushing her head down to my level but then she gets mad or scared and pulls back hard on the lead.
I wouldn't try to force her head down. With my experience this usually just freaks them out more and the result is them trying harder to get away. (Like what was happening when I brought my arm up over my mare's head.) Just coax her and try to keep her as relaxed as possible.
Over time she will relax and realize it is going to happen whether she likes it or not. No Dandy (my mare) will take the bit however I want to put it on --over her head or otherwise.
I hope this helps! If anyone see's something wrong with my information please speak up! I am not a professional, so I'm not saying this is the way to do it. Good luck!