कृतस्य करणं नास्ति मृतस्य मरणं तथा |
गतस्य शोचनं नास्ति ह्येतद्वेदविदां मतं ||
You can't undo what is already done. Just like a person who is already dead doesn't have death. There is no use of repenting what has already passed. This is the opinion of people who know the vedas.