Thank you for your question. I can certainly see that you love your cat, and you want her to be safe from all of the things that could harm her.
It can be frightening to see two cats fighting, but cats live by rules that are very different from the ones human beings follow. One of those rules is that a young cat always has to be submissive, or give in, to an older, stronger cat. That way, the more powerful cat will stop hurting, or even trying to kill, the younger cat. Martha’s instincts are right. They are telling her that when another cat attacks, she should NOT fight back because she has no chance to win.
You can not teach a cat how to fight. You can only protect her from harm. Martha will be safest if she does not have contact with more aggressive cats. She sounds like a cat who should have her own litter box and food and water bowls inside the house so that she does not have to go outside for anything. Martha would probably be very happy sitting in the sunlight and watching the world through a window in your house.
One other thing that is very important is that she is spayed (fixed) by your veterinarian. If she does accidentally get out of the house, other cats will leave her alone if she is spayed, and she won’t have the urge to wander away from home.
If you do these things to keep Martha safe, not only will she show her appreciation by loving you, but you will feel proud of yourself for taking such good care of her. Good luck to you and Martha.