§147

Another reason one might be tempted to say that understanding is a mental state - or at least that applying a rule is not a criterion of understanding - is that in your own case you surely don't know that you understand by having experienced yourself applying the rule.and what you have understanding of is a rule which applies to a series that is infinite, whereas any application must be finite.

Is the first person/third person asymmetry here like the case of pain? - No. In the case of understanding it makes sense to say "I think I understand" but find that you do not. In the case of pain it does not make sense to say, "I think I'm in pain", at all (except, perhaps, in cases of very mild pain).