Sefer Chofetz Chaim last part of chapter 4 halacha 12

The advice of the Chofetz Chaim is just not to speak lashon hara because if a person regularly speaks lashon hara then it is pretty much impossible to remember everyone you spoke badly about to apologize to them, and those that you do specifically remember you might be too embarrassed to tell them you spoke lashon hara about them if they don’t know you did, and then apologize. Even worse if a person speaks lashon hara about a family it might affect generations and then it literally is impossible to apologize to everyone. We already learned that Hashem won’t accept our apology to him if we don’t first apologize to the people we accosted, therefore the best thing to do is just not speak lashon harato begin with.

This answers the question he had on the Gemara Erechin 15b which seems to say that a person who habitually speaks lashon hara can never fix his wrong. Though everyone knows that Hashem allows anyone to repent even a person who decides to not believe in Hashem has the ability to regret his decision and do complete teshuva which Hashem will completely accept. A person who speaks loshon hara can’t be worse that one who denies Hashem’s existence?!

However the answer is of course anyone can repent to Hashem, feel remorse for what they did, admit, and decide to try never to do it again, and Hashem will accept the repentance with open arms, but when also sinning against one’s fellow man and not being able to apologize to everyone then Hashem can’t fully accept your apology to him and your repentance cannot be complete. In this way, in fact speaking lashon hara is worse than denying Hashem! That is why it is best not to speak it!!!