Harness Problems with my Aussie

Hi! I have a 4.5 month old Mini Australian Shepherd, she's about 15 pounds now. I've been trying to train her to go out and walk on her harness since she refuses to budge on her collar. So when I grab the harness to put it on her if she is already sitting down she will allow me to put it on her but she looks really sad and puts her ears back when I put it on. After it is put on she keeps her ears back for the entire duration of being on the leash. I'm trying to teach her to go to the bathroom when on the harness so we can start getting her to go in a designated spot in the backyard (I bring treats with me too if she goes). If she really has to go and is desperate she'll go potty when on the harness, otherwise she doesn't want to move around and sniff as much. If I walk her on her harness she doesn't do bad, she just gets distracted by a lot of different things like cars, neighborhood kids, and other dogs. I'm just wondering if it seems like I'm doing everything I should be doing or if maybe I should be doing more? Any help is appreciated!

To get her used to sniffing around whilst on the harness, try throwing one or two treats on the ground to give her the idea.

No matter what she is walked on, collar/harness/headcollar etc, she will get distracted at her age. It's just a case of time and training her to cooperate

Are you sure you've adjusted it properly? It may be chafing your dog if you don't have it on properly. That may be why she doesn't like it.

If it's not physically uncomfortable for her, she should get over it with a few treats and positive encouragement.

Not sure if this will helped but these are some tips I followed when using a harness to train my dog to not pull.

What about letting her know that leash isn't a sad thing? Try bunch of games that you can play with her and the leash! Playing with leash and having fun time will make her create a positive memories with the leash!