how to get my bottom denture to fit better?

You will most likely need to go to your denturist to have them re size them so they will fit properly. You cant change your jaw to fit the denture! Have to change the denture to fit your jaw.

If you don't wear your denture often, that could have something to do with it. When you first get a denture, they are made to fit perfectly customized to your jaw (if done properly, of course), but if you do not wear them on a regular basis, you actually lose bone mass in your jaw. This is because if you don't use your bones, as in, if there is never any real pressure on them, they resorb. Thus, losing bone mass, which in turn makes your denture loose. Good luck!