Where does gravity get its energy from?
So, I'm taking a high school environmental science class and we just started a unit on energy. We were talking about different types of energy and one was gravitational. I understand the law of conservation of energy and how it works with most types of energy but, from what I can tell, it doesn't seem to apply to gravity. It seems to me that, with everything that falls, there would be less energy going towards earths gravitational field because energy is used to attract the thing that falls and eventually earth will stop attracting objects from a lack of usable energy, but then a large ball of mass wouldn't have gravity, which also doesn't make sense.
Also, it seems that the more energy a black hole uses to alter the course of everything (especially light) would decrease it's pull because of the energy used to pull things in (no force was used to lift an object entering orbit) but it does the opposite. If all the energy comes from lifting the object then what about a rock that has always been at the top of a mountain since the planet was formed? How did gravity get the energy to completely reverse the trajectory of an object that was thrown in the air? In open space if you lift or throw an object it will keep going unless some type of energy changes that.
I understand that the energy from things falling comes from lifting the object but where does the energy come from when an object slingshots around a planet or moon?
Please remember that I am only in high school and use layman's terms!