Hi ((((((((((((Sherryanne)))))))))))))))
How old are you? Have you reached the "age of majority"? If you have, your parents (really) don't have any control over what you do for your own health and wellbeing. I'll tell you this much... my friend has been having pain for some time now. Last week she had gall bladder surgery to have it removed. Best decision EVER because she wound up in hospital with severe problems, and this was the only option. Tell your mother that you're risking your health and wellbeing and it will cost a whole lot more later to fix your health than dealing with it now.
As far as I'm aware, a person can function normally without a gallbladder, so you're not doing yourself any added harm by having it removed (and it's a fairly non-invasive procedure as well). You've also got one doctor's recommendations... is that your normal doctor? Perhaps seeing a third and if they happen to agree with what the first doctor said it would also help to convince your mother?
If that fails, or you don't want to see yet another doctor ... perhaps printing off resources about gallbladder removal and whatnot would help convince your mother.