dimitri harrell , 21 , was arguing with samirria white , 19 , over his ` infidelity '
with three-month-old daughter in his arms , he apparently pulled out a gun .
he then shot his fiancée in the face at close range , killing her , officials said .
now , harrell has been charged with single count of second-degree murder .
he revealed to police he had stored the gun under his daughter 's mattress .
also claimed he shot victim accidentally while pulling weapon from pocket .
harrell caused ` trust issues ' after having two children with another woman .
in custody on $ 200,000 bail ; if convicted , faces up to 40 years behind bars .