You may experience something internally that others cannot see. Just because you experienced internally does not make it any less real. Just because you cannot show it to others does not make it any less real.
Allah says He will show us His signs on the horizons and within our own selves. So we should seek these signs and if Allah wills, we will find within ourselves the signs that point to our Lord. And we should cherish those signs and never doubt them.
Those who are cynical, whether they are materialists, atheists, etc., might say that just because it is internal, it is not real since insane and mentally delusional people feel and believe many things that have no reality, so how can one know for certain that what one has experienced internally is not a delusion or a fit of insanity?
To this we can say: Why should the delusion of others mean anything to me? Why would the delusion of others detract from the veridicality of my experiences?
Does the fact that mirages and optical illusions exist mean that the eyes cannot be trusted?