1. I haven't seen him this morning. => It is still morning (i.e. it is not yet noon today)
2. I didn't see him this morning. => It is still the same day, but it is probably no longer morning. If it is still morning, the speaker sees any possible opportunity to 'see him' as already being in the past.
Hello Aunty really I am very sorry by oversight i haven't seen your messages don't mistake me. Pl. let me know your convenient time I will call you in that time today.