Proof in what exact way you were afraid, confused or frightened. Did you fear bodily harm? What made you think that? Did the intruder move to attack you? Did they notice you?
The law is written such that you need to prove innocence, that is, the existence of these effects. If you cannot prove you felt this way you are guilty.
as any reasonable person in this situation would
That is not sufficient. It is always scary to be put into a situation where the need to self defence arises. You must prove your terror was exceptional.
According to law student material of a university:
The asthenic affect [confusion, fear fright] must be of such severity that the perpetrator is able to process the events only to a significantly reduced extent.
