You've been gearing up for this for months (in addition to the 27 years).
It's the right thing to do and I know you can do it. Go in prepared with examples of how they helped you, times when you knew you could count on them, ie your "get out of trouble free" card. They will blame themselves and that's not your prime concern, but reassuring them they did good overall through the years will help them keep together and concentrate.
Getting the biggest secret out feels really good. Draining though - don't firmly commit to anything later that evening.
"Don't think it hasn't been a little slice of heaven just because it hasn't!" --Bugs Bunny