Backstabs you constantly in a subtle way

What do call call when someone Backstabs you constantly subtlely with you only realizing it after a fixed time because of the subtle nature and you always giving them the benefit of the doubt. The negative action causes you mental anguish.

Example: In a friendship situation what do you call it when someone is fake nice to you so you think they act in your benefit but in subtle ways they intentionally cause you mental harm e.g. they give excuses to you constantly and they do so intentionally and you realize they do so because they want to do harm to you

Words that don't quite fit.

Backstab - The word is a bit too harsh as it doesn't convey the subtleness of the behavior and it isn't a single event but constant behavior over a long time

Condescending - I want to convey more the fact that they are not overtly/obviously doing the negative action but it is done subtlety which cause harm since you only realize it after a long period because initially it isn't

Sly - This is probably the closest I can think of but I am looking for a phrase or words that convey the slyness and the negativity in a phrase like the phrase backstab does

I am looking for a word, phrase or expression to convey this in speech and writing

I usually refer to such behavior as "undermining":

undermine (verb) : 2. damage or weaken (someone or something), especially gradually or insidiously. "this could undermine years of hard work" synonyms: subvert, undercut, sabotage, threaten, weaken, compromise, diminish, reduce, impair, mar, spoil, ruin, damage, hurt, injure, cripple, sap, shake; informal: drag through the mud "their integrity is being undermined"

I would call this an insidious relationship.

Full of wiles or plots; lying in wait or seeking to entrap or ensnare; proceeding or operating secretly or subtly so as not to excite suspicion; sly, treacherous, deceitful, underhand, artful, cunning, crafty, wily. (Of persons and things.)

OED

The Domestic Abuse Bill aims to create a specific offence of domestic abuse that will also criminalise coercive and controlling behaviour. ... to identify some of the more insidious and damaging behaviours that perpetrators use to control their partner or ex-partner which are covered within the new offence.”

Police Professional, 03 jan 2018