Once you have studied about pointe shoe fitting and purchased the best fit you can get, you still might need to make some modifications.
Pointe shoes that are hand made have tiny variations in the finer detail of the shoe, and one pair may not feel as good a fit as another, even if it is made by the same cobbler.
If the shoes feel off a bit, there are many ways to make them exactly the right fit.
If you check the Dr. Scholls (or some similar foot care brand) area of a store you'll find all kinds of insoles, gel soles, corn and blister pads and other things you can cut up and put in your shoes, or on your toes, to make a pointe shoe fit exactly.
In fact, many professional ballerinas customize every pair of shoes that they have. They develop all kinds of tricks to make the shoes enhance their dancing.
Heel grips can help the back of your pointe shoe if it seems a little wider than usual, even though the size is the same as a pair you had that was a perfect fit.
Gel soles can be cut up and glued into the shoe or around or in between your toes, to get the perfect toe spacing, disperse the pressure on the toes in the box, or on the wings.
Plastic wrap is great to put in between the tops of the toes and your tights - you get a slide instead of the friction that rubs the weave of the tights into your skin.
If you have a bruised toenail, it takes a while to go away. A podiatrist may be needed to cut into the nail and drain the fluid from beneath it. Then, you may need a full sponge toe cover over the nail, and even a half size larger shoe, until the bruise has healed.
In other words, use everything that is available to dance safely, and know that you can avoid dancing in pain. Especially if you have an exam or a performance, you do whatever you have to do to dance your best. There is no rule against that.
Knowing about and getting the right pointe shoe fitting will make dancing in ballet toe shoes easier for you.