How can I prove that there is a decidable language which is not in P?
Unfortunately, this doesn't work at all. What you're proposing to do is to simulate a Turing machine on some input, and ask whether it accepts/rejects that input in polynomial time, and that question is not well-formed.

"Polynomial time" is the class&nbsp;P of Turing machines&nbsp;$M$ that have the following property: there is are constants $c$ and&nbsp;$k$ such that, for every input of length&nbsp;$n$, $M$&nbsp;halts in at most $cn^k$ steps. Notice that this property talks about all inputs, so you can't test it just by looking at one input.

Indeed, it doesn't make sense to ask if a particular input is "accepted in polynomial time." Let the input have length&nbsp;$n$ and suppose that the Turing machine takes $t$&nbsp;steps to accept it.  We can always find constants such that $t\leq cn^k$. For example, we could take $c=t$ and $k=0$.

You need to look at the time hierarchy theorem which says, loosely, that Turing machines with longer running time can decide more languages than Turing machines with shorter running times. Its proof does indeed use diagonalization, so you started with the right idea.