The ruling of fidyah is for such a person who is too old and unable to keep fast and there is no hope in future for the same. Or a person is caught in such illness which weakens his physical power day by day and he is unable to keep fast due to his extreme weakness with no sign of recovery and this condition continues to his death. Such person is exempted from keeping fast and may pay fidyah for each fast. In the question mentioned above, first you should consult any expert Muslim doctor. If he stops you from keeping fast then you may leave it right now but at a later stage if you are recovered and became fit to keep fast then the ruling of fidyah shall not apply to you. You will have to make the qaza of those fasts.
Allah knows Best!
Darul Uloom Deoband