finite difference bending for a fixed plate!

hi everyone, i have a question with regards to finite difference method. i have a 8 by 8 matrix ( for 7 internal nodes) that works fine when i am obtaining the bending moment of a simply supported plate whee the outer edges will have a bending of zero.

when i am trying to recreate the same matrix for a fixed plate (where this time the edges will have a bending of 2*last internal node along the x and z direction of the plate). i cannot figure out how i would program this and keep it generic so it works for every time i do different support conditions.

the programming fore the simply supported plate was the following;

%----------------BENDING X DIRECTION-------------------
for j=2:inc+3
    for i=2:inc+3
        momentxdirection (i-1,j-1)=D*(largedisplacement(i-1,j)-2*largedisplacement(i,j)+largedisplacement(i+1,j))/dx2;
    end
end

momentxdirection;

%----------------BENDING Y DIRECTION-------------------

for i=2:inc+3
    for j=2:inc+3
        momentydirection (i-1,j-1)= D*(largedisplacement(i,j-1)-2*largedisplacement(i,j)+largedisplacement(i,j+1))/dx2;
    end
end

where the large displacement is the deflection of the plate, which would not need to be altered since it comes out correctly.

the only thing that would need altering is the bending in x direction and bending in y direction programming.

help!