I am also considering the mirena iud to help treat endometriosis. It is scary, especially considering I haven't had kids or anything. The thing with the mirena is that the hormones are the highest in your system for the first three months, and then the hormone levels balance a bit better. So if there are side effects, they are more likely to occur in that space of time. Most people who do have a reaction, like break through bleeding, have it settle down after three months. Also, everyone seems to have a different reaction, so it's hard to predict what will happen.
stay positive, and I hope everything goes well for you!