I would suggest that you speak to your new company regarding the challenges you are facing and explain the reasons why you may not be able to continue in your job. Help them understand your situation and make a smooth exit. This approach could create a win-win situation for both parties, as it is detrimental for the company to have an employee who lacks interest in their work. Not only would this result in zero contributions, but it could also harm the company in other ways.
However, I believe that your actions towards your previous organization were completely unethical and unprofessional. It would have been better to inform them about your new opportunity. The professional network is relatively small nowadays, and the HR department of your previous company is likely to find out eventually. Therefore, it is essential to be prepared with a convincing explanation.
Wishing you all the best!