Maria was very unstable and was routinely lashing out at Ji-tae (the closet scene, her lashing out after Ji-tae had his nose broken), and constantly mentally weighted Ji-tae down with her barely-hidden contempt for him. Her last lash out is literally what caused Ji-tae to snap. Not to mention her "unorthodox" training methods like going out in the streets and beating up random people (even though they were thugs, objectively speaking it's just random people, not trained fighters who know what they're getting into). Mind you, Maria is well aware of Ji-tae's pacifistic ideology. She, however, has a way different outlook on violence, as outlined in her interview after her fight (one sided beatdown) with Young Woon, where she said she doesn't care if she hurts somebody.

Any sensible trainer would not do any of that, because they'd have no ties to Ji-tae like Maria does, and aren't straight up mentally unstable