if h2o2 has interamloecular h-bonding then why is it very unstable

The formula for hydrogen peroxide is H2O2, so compared to water (H2O), there is an extra oyxgen atom in the molecule. The oxygen atoms in hydrogen peroxide are not in the most stable state. They have a tendency to get the most stable state,In the case of the oxygen atoms in hydrogen peroxide, they have an oxidation number of -1. This is not the most stable oxidation state for oxygen. The most stable oxidation number for oxygen is -2 (as is the case for the oxygen atom in water), so the oxygen atoms tend to want to gain electrons to reach this oxidation number, which is the most stable state for room-temperature oxygen.

In the process of gaining electrons, the oxygen in hydrogen peroxide is reduced, and the way it gains electrons is by oxidizing something else
Oxygen has very low energy atomic orbitals. That is to say, it is a very electronegative element. In laymans terms, it wants more electrons. You have a simple molecule where two very electronegative atoms (the two oxygen) are fighting over the electrons they have between them. This is one way that hydrogen peroxide can oxidize other molecules.Another place it can get these electrons is from another molecule of hydrogen peroxide. When it does this the following reaction occurs.

2H2O2 --> 2H2O + O2

Since oxygen is a gas it escapes and the reaction is entropically pushed forward.