Is what happened permissible, that the employer hired and trained a new employee, who then turned against me, and was supported by the manager, after I had worked sincerely for the sake of Allah?

We advise you to be frank with your manager about what you feel within yourself, as it might be a misunderstanding or an illusion. You must perfect your work and be sincere to Allah, for "Indeed, Allah loves that when one of you does a deed, he perfects it." And treat your colleagues well, for "Is the reward for good [anything] but good?" And "Indeed, those who have believed and done righteous deeds - the Most Merciful will appoint for them affection."