S.P.
Hi Kristen,
First of all, hang in there!! I am in the same position as you right now. I have twin girls who just turned three. They are in underwear and in general are doing very well with potty training. But occasionally they do have accidents and there have even been days when one of them seems to completely forget how to use the potty! Just a few days ago she wet herself 4 times before lunch. But you do need to just stick with it, do not go back to diapers. Stay persistant so they know this is how it's going to be, they need to use the potty and that's it. I do believe the weekend thing is because her schedule is different. I find myself not as diligent with telling my girls to sit on the potty and they forget to go because things are more hectic, more people are home and exciting things are going on. I wouldn't worry too much, things will work out. Maybe you could try setting a timer for an hour to remind you and her to have her sit on the potty.
As for her holding it, I would definately try to relax on that issue. It sounds like a definate power struggle is in the making. Just set specific times for a "potty break". That's what I did with my girls when they started to fight me. Tell her it's time for a potty break and have her sit on a potty. If she sits for five minutes or for 30 seconds, leave it up to her. She will get tired of being wet and if she sees you are not making a big deal out of her not going, she will change. Just make a really big deal when she does go. This is what is working for me. We also do a star chart to help them see what a good job they are doing. Hopefully you will find something that works for you.
You are not alone!!! This is the hardest part, I think, and definately the part I absolutely hate! But it too will pass, just be strong and keep telling yourself, noone goes to high school in a pull-up! Good luck!