Is saying, "Why all this suffering, O Lord?" and similar كلام (words) – due to the severe illness that afflicted my daughter – considered disbelief in Allah, and what is the way to repent from that?

A Muslim must submit to Allah’s divine decree and destiny, and avoid discontent and despair. For if Allah Almighty decrees something for His servant that they dislike, such as illnesses and calamities, He intends good for His servant through it. Calamities are trials and tests from Allah to reveal the sincerity of the believers. Therefore, one must entrust their affairs to Allah and not object to divine decree, but rather seek forgiveness and supplicate to Allah to remove the affliction.