I know that in quantum mechanics the stronger the force, the shorter the range it mediates between (maybe I have problem in this postulation, if so please correct me). So how can gravitational forces be much weaker than electrostatic forces while both the graviton and photon are massless?
Maybe I have a problem understanding the relation between boson properties and the strongness of the force it mediates, can someone explain to me please if I have a problem with the basics.
Massless (gauge) bosons only mean that the force is long-ranged, but have nothing to do with how strong the force is.
Understanding why the gravitational force is much weaker than the electromagnetic force (or any forces in a gauge theory) is still an open problem, but attempts to understand this are usually referred to as the "Weak Gravity Conjecture" (WGC). Here is the original WGC paper. There have been more recent attempts to derive the WGC from sensible physical principles like unitarity, causality, etc.