I keep getting holes in one side of my model when I use the mirror tools setting. How do A) I keep this from happening and B) fix it when it does?
I don’t think those are holes. I believe it is the opposite side of your mesh. Kind of like turning your shirt inside out, except your shirt would intersect with itself.
For me, this seems to happen when trying to add/subtract too much. If you remesh it, it should go away - although you may loose a little bit of your form.
Yeah either re-mesh or probably more useful would be to turn on dynamic topology so that you can add fine details where you need it without exploding the complexity of your whole model. Re-mesh when you are just getting the general shape of your model, and turn on dynamic topology for adding in the fine detail.