When Sherlock replaced the lid on that coffin…with the words I Love You taunting him…he knew he was, for all purposes, burying what relationship he had with Molly. That I Love You was a stamp of closure and the end of what relation he had with Molly, as he knew they could never go back to any earlier equation after that call.
Then his decision to refuse that conclusion, his saying No!
Sherlock destroyed that coffin because he refused to accept that conclusion.
He accepted what he felt for her, acknowledged what she felt for him, and for all that he was worth, he was not going to call quits.