A star is held up by the energy (actaully radiation pressure) given off by the fusion reaction in the centre. A young star is mostly hydrogen which reacts rather well, as a star ages it uses up hydrogen in the core, then Helium, Carbon etc until it gets to a material that won't easily react. As there is no more energy it then collapses.
A star loses a negligible amount of it's mass through most of it's life - mostly charged particles in the solar wind. Remember the fuel that is used in the reaction doesn't get used up, it just converts to helium which stays in the star.
Cepheid variables are a good example. They are relatively massive stars that cyclically collapse and expand. During the collapse phase, they reheat, and the reheating causes them to expand once again. This cycle can continue for billions of years.