If she is still a friend, she has already learned to forgive the mistakes you made.
I wouldn't create an unhealthy guilt in myself by thinking that I need to rehash things that are long past. She probably knows that you had some mood swings in your teen-age years.

At best, I might mention in a letter that you regret all the teen-age problems of youth, but that you've grown a good bit in your understanding of yourself.

Just let her know that you value her loyalty and hope to be as kind as she has always been to you. I don't like the "guilt" thing and see no need to dwell on it as difficult for you.

You must remember that the teen-age years are like trying on outfits to see which one fits us best. A more mature person would think as you do, realizing that the
feelings were all part of moods during those early years.

Take care and forget about guilt. I don't know that having guilt is helpful in this
instance.