ans =
Why do you want to use code for partial differential equations if you have an ordinary differential equation to solve ?
syms psi(x)
eqn = diff(psi,x,2)+0.2*diff(psi,x)+psi+0.2*exp(-0.2*x)*cos(x) == 0;
Dpsi = diff(psi,x);
conds = [psi(0)==0,Dpsi(0)==0];
simplify(dsolve(eqn,conds))