You can try to get her used to the harness and use positive reinforcement which will hopefully make her feel neutral about it, making it easy for you to put it on.
This is what I would do. Sometime when you have a lot of free time in a day, on a weekend or something, start a mini training session. Put the harness on the floor somewhere, then using pieces of dry food (or little bits of cookie bones if you don't use dry food) ask her to sit next to the harness, lay down, give paw... whatever commands she knows and give lots of praise. Staying calm and positive is very key, dogs pick up if you're feeling anxious!
After a few commands, if she's doing well, try putting the harness over her head. Not even putting it on correctly, just draping it on her and do some more "sit" commands, rewarding with food to calm her down.
If she gets anxious when you add the harness, just be patient + don't try to force it. Eventually you should be able to put the harness on the correct way. Try having her wear it around the house for a while (an hour'ish) then go for a walk outside.
It may take days for it to work, but pe patient!
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