  We present the first simulations of the formation and feedback of massive
stars which account for radiation forces as well as photoionization feedback
(along with protostellar outflows). In two different accretion scenarios
modeled, we determine the relative strength of these feedback components and
derive the size of the reservoir from which the forming stars gained their
masses.
  We find that protostellar outflows alone limit the stellar mass growth only
in an accretion scenario with a finite mass reservoir; when including accretion
and ram pressure from large scales (> 0.1 pc), protostellar outflows do not
limit stellar mass growth at all. Photoionization and HII regions dominate the
feedback ladder only at later times and only on large scales. Specifically,
photoionization yields a broadening of the bipolar outflow cavities and a
reduction of the gravitational infall momentum by about 50 percent, but does
not limit the stellar mass accretion. On the other hand, we find radiation
forces restrain the gravitational infall toward the circumstellar disk, impact
the gravito-centrifugal equilibrium at the outer edge of the disk, and
eventually shut down stellar accretion completely. The most massive star formed
in the simulations accreted 95 solar masses before disk destruction; this mass
was drawn-in from an accretion reservoir of approximately 240 solar masses and
0.24 pc in radius.
  In the regime of very massive stars, the final mass of these stars is
controlled by their own radiation force feedback.
