inertia is mass and vice versa, momentum is mass or inertia x velocity
A moving body has momentum and kinetic energy, under the laws of conservation of energy, for the body to lose momentum, or velocity you have to move some of the kinetic energy into another form of energy.
Drag converts kinetic energy into other forms of energy such as heat, noise and kinetic energy in the air.
when the thrust equals the drag the body remains at a constant velocity.
Reduce thrust and the body will lose kinetic energy thru drag as heat etc, and will slow down until once again the thrust equals the drag. and we are again stable.
Now if you are in a glider the only thrust you have is in fact the momentum you have which is your mass x velocity in straight and level flight, or you can fall to earth and use G to give you acceleration which will again increase your velocity and hence your momentum. Or you can use a thermal say and raise your height increasing your potential energy which you can then exploit to convert it to kinetic energy once again, only to lose it as drag in the form of heat etc.
The key to understanding this is the conservation of energy and managing the energy available in whatever form you have it, it could be chemical energy in your fuel, or potential energy in your altitude or kinetic energy in your velocity and mass. BUT once you let it out of your system into another system in some form no use to you such as hot air or noise its very very hard to get it back to do work for you.
I'm not a pilot, i just design the hot bits that push you guys along, but i am pretty sure i'd rather have some altitude and speed in reserve as energy stores, just in case my bits let you down at the critical moment.
GB