Is she happy wearing the harness? It might be why she resists when wearing it.
You could try popping the harness on her in the house then just carrying on as normal so she gets more used to it. Just start by putting it on her for a few minutes at a time and throw some treats around then build the time up.
She will learn then that wearing it leads to good things.
You could even try clicker training and clicking when she is at heel to reward the position but she will need to be tuned into the click first.
Best of luck!