Horse is either not as saddle broke as you think (and yes, a lot of horses you will be able to get on bareback with no problem, but they haven't had experience with the saddle), or sore, or was testing you out when you got her and now she's gotten very much wise to your inexperience and is playing on it.
First thing to do - get her checked out by a vet to make sure there is no pain issue.
You can't train the pain out of a horse.
Find someone experienced - that doesn't mean the 15 year old kid from up the road that claims to be an expert. It means a mature person with years of experience and the results to back that up - to help you. Chasing the horse around with a saddle is extremely pointless. You'll always get tired before the horse will, and every step the horse takes away from you, she is winning the 'game'.
Right now, you are training your horse to run away from the saddle.
I don't believe the horse is scared to be honest, I believe she has 100% pulled the wool over your eyes. She thinks she's the boss of the place and you are allowing that to happen.
Without getting some good, solid, professional assistance, this behaviour with compound into a lot more of a problem than you will be able to deal with. When left unchecked, a simple problem can manifest into a dangerous problem.
Please, find some help.