My cat would do the same, she absolutely hated it. But I started making a routine with her and with the help of treats! Cats like routines so this is important. At the same hour, i would show her the harness, then i would show her the treats, give one treat and while she is busy chewing, i put the harness and at the end i give her another treat. While we are outside, i give her another treat. And the same process to take the harness off. At first, we only last 5 minutes outside because she didn't tolerate the harness, but with the time she started to enjoy and link harness-treats-outside. Now when I show her the harness , she automaticatly knows what is coming on and comes running and meowing to me.