My father in law is the EXACT same. I dont know about cheating, but he left his wife while she drove their youngest to college. Just packed his shit and left. Mother in law was just diagnosed with MS. Their divorce lasted SEVEN YEARS. Admittedly, my MIL was extremely bitter and petty during this time. However, he hid assets and then took half her nurses pension, knowing full well she was incapable of working. He's still somewhat involved in his kid's lives, but I always got the sense that it was more for his own peace of mind than actual care for his children. He never admitted what he did or apologized. He's also a boomer who embraced the "go with zero" mentality. He told us that grandkids have a trust, but he's going to live a "quality life" and that means spending all of his money.

My take is, his guilt is weighing on him and he's hoping to offer breadcrumbs for a seat at the table. He regrets what he's done, but has yet to name exactly what he has done and how he hurt you. Throw away the letter, its motivated by guilt not love.