I'll wait for somebody else to comment about the leash training because our pup has never struggled with that issue, for us it was as simple as using roast chicken (high value treat that she only got during walks, and she was really motivated by it and so I had her attention during walks).

The potty issue, I HEAR YOU! My pup is 9 months old and last night was her first dry night ever. I've been so consistent with the potty training, I've done it right but it still took a lot of time and we still aren't there yet, but we are making big progress now. We just never had that 'click into place' moment yet.

I can only advise that you stick with it, think of the leash training and the potty training as a piggy bank with your jackpot. Every time you stay consistent (like taking the pup outside the same time every day) or you manage to get her to potty outside, its another coin in the piggy bank that will add to your jackpot goal of having her trained. Keep motivated its frustrating to take one step forward and 5 steps back when your pup gets something wrong, but you can do it. Id definitely advise getting a harness for your dog too, it's much harder for a dog to bite the leash when it's clipped away from the collar and its easier for you to keep control of the dog too.

I have a Yorkie and the vet said that they are by far the hardest dogs to potty train because of small bladders and that they're an impulsive, live in the moment type of character, which is true! We don't struggle with anything else with her right now, bit the pottying is a HUGE issue and my husband is furious, although he doesnt do anything to help and will just let her potty inside when I'm not home because it isn't "his job" so its definitely a 1 step forward and 5 steps back kind of thing.

The brushing I can probably help with, because a Yorkie needs a lot of brushing and it was as easy as using a decoy hairbrush! I would start with brushing her when she was relaxed with an old hairbrush I didn't mind getting destroyed, and when she went to bite and play with it, I'd pull another brush from my pocket and brush her while she played with the other one and was distracted enough. If she already has matting behind her ears I'd recommend finding a good groomer and explaining how you're struggling to do it- our groomer is doing social distancing pickups and drop-offs to groom her dogs at home.

Hope I helped a little, you can do this!