A question about my young horse, bad habits??

So my 5 year old mare has recently been developing some bad habits. She is fairly green, and I have been really trying to keep everything consistent in regards to working with her. However, in spite of all my efforts, she seems to be completely ignoring me. She doesnt really respect me, although I am an ertive rider and handler and I don't let her get away with things. She can be pushy, and althouhg I can handle her it gets really tiresome to have to be constantly schooling. =/ I do a lot of groundwork, and as its been REALLY rainy where i live lately that's about all ive been able to do. She is farily good on the ground and longes pretty well. She works wonderfully at liberty and is stuck to me like glue =) She know voice commands (whoa, back, over, etc). However, all of this seems to come and go. Sometimes she listens, sometimes she doesnt. Its supremely frustrating. And that leads me to my biggest problem........every time I go to saddle her up, she bites. Like a lot. and pretty viciously. She's never actually made contact or hurt me at all, and every time she does this I make it clear that it's not okay (circling, backing, a sharp smack on her neck or nose. thats the only time i ever hit her). I'm doing everything in my power to combat this and stop this bad habit from settiling in but NOTHING IS WORKING. She bites when she's upset, or doesnt want to do something and I'm afraid that it will get really dangerous. I'm trying to rule out saddle pain but it's fairly difficult to discern between pain and her not wanting to be worked or ridden. Sometimes when i groundschool and she decides she;s done, she satrts to bite and get really nasty abouut it. Everything seems to be going downhill, even though Im doing trying my very best not to let it. I'm really stumped with this one. I am expereinced with horses but I'm feeling like I need some extra advice with this one. I know that in this situation a lot of people would consider sending their horse to a trainer, but I would really hate to do that. Even so, any information on that option is appreciated. So if you have any ideas or suggestion please answer!!!