  Bulk FeSe is a special iron-based material in which superconductivity emerges
inside a well-developed nematic phase. We present a microscopic model for this
nematic superconducting state, which takes into account the mixing between
$s-$wave and $d-$wave pairing channels and the changes in the orbital spectral
weight promoted by the sign-changing nematic order parameter. We show that
nematicity gives rise to a $\cos2\theta$ variation of the pairing gap on the
hole pocket that agrees with ARPES and STM data for experimentally-extracted
Fermi surface parameters. We further argue that, in BCS theory, $d_{xz}$ and
$d_{yz}$ orbitals give nearly equal contributions to the pairing glue, i.e.
nematic order alone accounts for the gap anisotropy, but has little effect on
$T_{c}$. This result questions the validity of the concept of orbital-selective
pairing. Self-energy corrections, however, make $d_{xz}$ orbital more
incoherent and reduce its contribution to pairing.
