Am I sinful for an amount I secretly took from the company and now regret, given my inability to repay it at present?

What you did is embezzlement and an infringement on the company's money. You are obliged to return it, for the Prophet, peace and blessings be upon him, said: "The hand is responsible for what it takes until it returns it." However, if the person who absolved you of the amount you took had the right to do so, then there is no blame upon you. But if he did not have the right to absolve you, then it does not waive your responsibility, and you must return it along with repenting to Allah.