Trust between horse and owner or rider takes time to develop. It can start with being friends. IMO all the emphasis on horses keeping their distance that all the gurus seem to preach has caused people to distrust horses intentions.
IMO people don't need to worry so much about a horse always obeying the human. Just hanging out with a horse can lead to a bonding. Spending time brushing, and braiding a horse can help with bonding too.
The more one spends time with their horse, the more one can understand the horse.
That being said, an untrained 8 year old Arabian mare is going to be more difficult for you than a kid-broke 10 yr old QH gelding would be (generally speaking)
I have rarely ever seen a horse maliciously attack a human for no reason. The chance of that happening are really low. No way to know what the mare was thinking because we didn't witness it, but I have seen horses put their ears back and snake their heads out shaking it and it didn't mean they were planning to attack me.
Actually, the times I have seen a horse get the most angry when I am standing in the stall with them...is when another horse walks by outside the stall. In other words, the horse is not angry at me, it is threatening other horses not to come near. They definitely are meaner to other horses when I am with them.
I call it jealousy...
Again, you need time, lots of time to build a good relationship with your mare. Try to just enjoy her. Say sweet things to her. Brush her and fuss over her. Give her treats. She will soon look forward to seeing you arrive. Once you witness that joy of your horse being thrilled to see you, your anxiety will lessen.
Speaking of lessons, take lessons and if needed send your mare out to be saddle broke, then learn to ride her.