Truth, or better, not telling lies, apart from being one of the fundamental moral obligations is a powerful way of getting yourself out of trouble. If you self-impose not to tell lies, you’ll have to do the right thing because you know you won’t have the false but easy way out lies provide. You think it twice before bad-behaving.
Idem with relationships, and this is direct advice to my niece (my son’s already heard it from me). You don’t break up with your boyfriend today and call him on the morrow. You don’t come back to him after he’s ditched you once. Either way should be a cease and desist. No second thoughts, no words of tenderness and forgiveness, no working it out —just plain no coming back. You, or he, think it twice before splitting out.