cats

You did way to much at once. It took me almost a year to train her. I started with just pulling the harness over the head. On and off. For some cats you need to start even slower and just give treats while they see the harness. (Or figure out what kind of things count as a reward for them, not all are treat motivated.) Eventually when the first step worked well I put on the full harness. Lots of treats and a few seconds then minutes until it's off again. Gradually lengthen it until she wore it indoors with no complaints for like an hour or two.

Second training thing done in the same time frame was get her used to her backpack. It's a pet carrier that magically was always full of treats initially. Eventually she had to go fully inside to get them. Them I zipped it close briefly for the first time. Eventually we graduated to her sitting (without a harness) in the closed carrier hidden in my flat while my front door was open. Over the following weeks o moved the carrier closer and closer to the door until we sat on our doorstep together. Then I trained lifting the carrier up and carrying her around indoors. Eventually I carried her outside without a harness onto the grass patch in front of where we live. We stayed 10 minutes and she spent the next two weeks mostly asleep digesting all the new sights, smells and sounds. Eventually she told me she wanted out so the next outdoor after that was with harness and leash.

Throughout the process you need to watch your cat closely. They are not an "it", they have fears and insecurities that you need to account for and go slower for. They like routine and don't care much about pleasing you. Dogs do, that's why it's much easier with them. For a cat you need to earn your right to walk beside her. Let them know you hear their complaints and work to alleviate them. That builds the trust you will need outdoors.

My friend's cat doesn't mind a leash, but I think she started when he was a kitten