What is the religious ruling on backbiting (Gheebah) and slander (Nameemah)? What are the consequences for someone who speaks ill of others? Is seeking forgiveness (Istighfar) and sincere repentance (Tawbah Nasooh) sufficient to expiate these sins?

It is recommended to avoid excessive talk that brings no benefit, because the pitfalls of the tongue are many. You have fallen into backbiting and ill-suspicion. Backbiting is mentioning your brother with what he dislikes, and ill-suspicion is forbidden, just like ill-speech. So, it is forbidden to speak to yourself and harbor ill-suspicion about your brother without clear evidence.