If you did not tell them nor any one else about your bad thoughts, you don't need to tell them now but just to seek forgiveness from Allah to forgive you and them as well.
You need to seek forgiveness and pardon from them. If you are unable to do that then seek forgiveness from Allah to forgive you and them and keep praying for them.
Although you might have been young, but you must have known that talking ill about someone is a bad thing. If this person is still alive, and you do see him, greet him, and just say 'forgive me if I have wronged you', without going into details.
I am sure he wont comment on anything, and answer in a positive way, and your conscience will be clear, in shaa Allah.
Thank you for your question. Even when we repeatedly promise and break our promises we can only keep seeking forgiveness from Allah, with renewed intention of not repeating that action. Sometimes it is hard to do something and we repeatedly fall back into the same mistakes. But just keep trying and begging for help and forgiveness. As long as we don't take our promises lightly and truly mean them, there is always a chance for us to not break them the next time.
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