When my Stepfather split up from his first wife, his adult daughter rejected him lock, stock and barrel. He was never given the chance to explain why he felt the marriage had failed. All his cards to her were never acknowledged and when she had kids, gifts were returned. Eventually he gave up. However, when he had a stroke, I tracked a friend of his daughter down and asked her to let her know. Within 24 hours she had contacted me and made arrangements to visit. When he died 3 weeks later, my step-father had met all his grandchildren and made peace with his daughter.
At the funeral I spoke to her and she said that at first she had been so hurt on her mother's behalf who put pressure on her to abandon her Dad. She eventually realised how hard it was for her Dad to stay married to her Mum after her Mum moved in with her but felt she had behaved so badly she didn't feel entitled to contact her father. Naturally she was devastated for the years missed but both of them were so pleased they had reconnected.
Send your letter OP, don't let regret haunt you.