I guess if I wanted my cat to get used to a harness outside, I'd first start with putting the harness on her inside and let her get used to the harness itself. After some days or weeks, or however long it takes, then take her out, but maybe put her down and sit next to her, just talk to her calmly so she knows you're right there and things are ok. Of course, if it were my cat and she meowed at the door to go in, I'd let her in, and kinda be happy she wants to stay in. Just move at her pace. Not sure why you're pushing for the outside thing, but in any case, I think it's best to do things slowly and gradually with cats, especially when it involves changing their comfort zone. We do it with food changes, with introducing new cats, with changing types of litters. Little by little seems to work best with a lot of cats, and it sounds like yours would benefit from a slow process of getting used to the harness, then getting used to being outside with the harness. Good luck!
EDIT- I just realised you said your cat IS used to the harness indoors.. (oops! Sorry

) So.... yeah, I guess it's time to be slow and gradual about the outside part, taking her out, but letting her in when she wants in, so she gets comfortable in knowing she's not stuck outside.