Why is my mare so feisty all the time?

I have a rising 5 year old 14hh connemara X New Forest mare. I've had her for a year. During that year we have had many ups and downs but recently (over the past 2 months ish) she's been extremely bad tempered and stubborn under saddle. She bucks constantly and refuses to do anything apart from jumping. I've had her back and teeth checked and there is nothing clinically wrong with her. Her tack fits too. I've been told that I need to get tougher with her but it just makes the situation worse and I end up having a long battle with her. The only way which I can get her remotely calm (but still fast and strong) is to lunge her vigorously before riding. Please help me, I don't know what to do as I love the pony too much to sell!