if something has long since happened, it happened a long time ago:
I've long since forgiven her for what she did.
WORD CHOICE: WORD CHOICE: since, for, during, overUse since to say that something started at a point in time in the past, and is still continuing• He has been living in Leeds since 1998. • We've known about it since May. Sinceis usually followed by a time expression ('last year', 'this morning', '4 o'clock' etc) or by the simple past tense. Use the present perfect or the past perfect in the other clause• I have loved movies since I first went to the cinema.• He had been seriously ill since Christmas.!! Speakers of British English usually say it is a long time/two weeks etc since..., and speakers of American English it has been a long time/two weeks etc since..., but both uses are correct• It's weeks (BrE)/It's been weeks (AmE) since I saw Grandma.Use forwhen you state the length of time that something has been happening• We have known each other for ten years (NOT since ten years). • I had been waiting for hours (NOT since hours).• I haven't seen him for ages (NOT since ages).During and overare used when you state the period of time in which something happens or changes• During her first year at college, she had several boyfriends. • Over the last six months, crime has doubled.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.