Finding the region of attraction using a Lyapunov function I'm trying to find an estimate for the region of attraction of an equilibrium point. The notes from Nonlinear Control by Khalil suggest that defining
$$
V(x) = x^TPx,
$$
where $P$ is the solution of
$$
PA+A^TP=-I,
$$
will yield the best results for an estimate. It also assures that the estimate can be found from these types of Lyapunov functions for exponentially stable equilibrium points.
The system I am studying is defined by
$$
\begin{gathered}
\dot{x}_1 = x_1 - x_1^3 + x_2 \\
\dot{x}_2 = x_1 - 3x_2. 
\end{gathered}
$$
The linealization matrix around the equilibrium point $x^* = \{\frac{2}{\sqrt{3}},\frac{2}{3\sqrt{3}}\}$, is
$A = 
\begin{bmatrix} 
-3 & 1 \\
 1 & -3\\ 
\end{bmatrix}
$,
and so,
$P = 
\begin{bmatrix} 
3/16 & 1/16 \\
 1/16 & 3/16\\ 
\end{bmatrix}
$.
Given the previous values, I took the derivative of V(x) w.r.t. time and substituted the system in the equation (did it in Mathematica to try and simplify it as much as possible), giving
$$
\dot{V}(x) = \frac{1}{8}\left(4x_1^2-3x_1^4+4x_1x_2-x_1^3x_2-8x_2^2\right).
$$
Now I need to find a region where $\dot{V}(x)$ is negative definite, but when I try to use inequalities with the norm of $x$ I get only positive norms raised to a power, and so $\dot{V}(x)$ can never be negative definite.
Using
$$
|x_1|\leq||x||, \quad |x_1x_2|\leq\frac{1}{2}||x||^2,
$$
I arrived at
$$
\begin{gathered}
\dot{V}(x) \leq \frac{1}{8}\left(4||x||^2+3||x||^4+2||x||^2+\frac{1}{2}||x||^4+8||x||^2\right) \\
\leq \frac{1}{8}\left(14||x||^2+\frac{7}{2}||x||^4\right).
\end{gathered}
$$
As you can see, it never is negative.
Am I being overly aggresive with converting everything to norms? How can I find the region where $\dot{V}(x)$ is negative?
Thanks in advance.

Correcting an error done.
In my previous answer (this is the edition of changes) I considered
$$
\dot V = (x,y)P(x-x^3+y,x-3y) \ \ \text{translated to}\ \  (x^*, y^*)\ \ \text{which is wrong}
$$
The correct procedure is to consider
$$
\dot V = (x-x^*,y-y^*)P(x-x^3+y,x-3y)
$$
thus giving
$$
\dot V = -\frac{3 x^4}{8}-\frac{x^3 y}{8}+\frac{5 x^3}{6 \sqrt{3}}+\frac{x^2}{2}+\frac{x y}{2}-\frac{4 x}{3 \sqrt{3}}-y^2+\frac{2 y}{3 \sqrt{3}}
$$
thus we have in light blue the region with $\dot V\le 0$ and in black the contour lines for $V$. As we can observe, for the choose Lyapounov function, the small attraction basin is shown by the tangent inner level curve.

