Why doesn't my backward Euler-method end up with the same result as ODE45?

I won't end up with the same result when using this backward euler method as when using ode45, I want to believe that my code is correct. But obviously it isn't. Would be very grateful for some advice concerning the code below.

 function [t,U]=backwardEuler(f,I,U0,N)

t=linspace(I(1),I(2),N+1); %Generates time nodes(?)

U=zeros(1,size(t,2)); %Vector in which the solution is saved

U(1)=U0; %u0
k=t(2)-t(1); %Step size
tol=k^2; %Fixedpoint tolerance

for n=2:N+1
    Uguess=U(n-1); % First guess, previous value
    g=@(x)(U(n-1)+(k*f(t(n),U(n)))); % Fixedpoint function changes every lap.
    U(n)=fixedpoint(g, Uguess , tol);
end
U=U'; % Same form as ode45...
t=t'; % Same form as ode45...

-------- fixedpoint -------

function x=fixedpoint(f, Uguess , tol)

x=Uguess;
while abs(f(x)-x)>tol
   x=f(x);
end

-------- f ------

function y=f(x,t)

y=-x-(t^2);