A term for someone who demotivates

What do we call a person who constantly demotivates others?

For context: someone who thinks he is so brilliant in English, keeps discouraging me about the questions/answers I post.

Hypercritical

FTD inclined to judge too severely; "hypercritical of colloquial speech"; "the overcritical teacher can discourage originality" also Excessively and unreasonably critical, esp. of small faults.

M.W says: criticizing other people or things too strongly or too often

A person who is hypercritical of everything one does, says, or writes can be extremely disheartening and demotivating. Think of a hypercritical parent, partner, teacher or manager whose exacting and severe behaviour can actively discourage initiative and creativity in their listeners.