Sometimes when I speak English, I feel like I lack the words to accurately describe my feelings or actions

Russian words that don’t exist in English

A person who did something great. 
Similar meaning to English "good job", "well done" but this is praise specifically for the person, for how good he is, rather than for his work.

Great spiritual suffering without any particular cause. 
“melancholy”, “sadness”, “yearning” -  similar but still not the same

"а stab in the back"

A negative moral quality of a person, as well as a deliberate action aimed at causing harm or humiliation to another person using deception, betrayal or selfish motives.

You can say: подлый человек

A bit similar to rudeness but still different. 
Rudeness is a rude, disrespectful attitude towards others, based on insults and aggression.
Хам - a person characterized by this behavior