Apologizing to someone you were mean to years ago feels weird. It can still matter a lot to them.

The weirdness is almost all on your side. You have the awkwardness, the hot face, the worry about dredging something up, while the person on the other end may have been carrying the thing for years with nobody ever acknowledging it happened. Yours lasts a minute; theirs has had a decade.

So keep it short and aim it entirely at them. Name what you did, say it was not okay, and say nothing about how hard this message was to send. Do not ask for anything back, including forgiveness, and let whatever they do with it be theirs.