Why is this second order system difficult to control?
\$\dfrac{1}{(s^2-1)}\$ factorises to \$\dfrac{1}{(s+1)(s-1)}\$, which has a stable pole at \$s=-1\$ and an unstable pole at \$s=1\$.

If this is the open-loop TF (you don't say if you wish to close the loop around this TF), then it can be stabilised by placing a zero at \$s=-a\$, giving an OLTF:\$\dfrac{s+a}{(s^2-1)}\$, and a CLTF: \$\dfrac{s+a}{s^2-1+s+a}=\dfrac{s+a}{s^2+s+(a-1)}\$, which is stable for \$a&gt;1\$.