If you are on pension and the amount is barely enough for your own basic expenses, then you need not worry about giving fidyah. Allah says,

"Allah does not burden any human being with more than he is well able to bear: in his favour shall be whatever good he does, and against him whatever evil he does...(Qur’an: 2: 286)

The Prophet, peace be upon him, also said, “Whatever I command you to do, do it as much as you can.” Since giving fidyah is beyond your means you are excused. So you need only to ask Allah to forgive you.