Dear M,
How are you doing? I hope you're well, if somehow, you've read this in this digital forum/website, I'm glad that you're happy. Our past may not be as pleasant as your present, but I hope you find this letter well. Guilt and conscience haven't been good to me; I'm always reminded of my poor delivery of both situations that led me to horrible endings, I was only trying to keep you both.
Friends as you may say, you haven't gotten over your ex at that time, and I told you there are no shortcuts in moving on. However, stubborn as you are, you still went for the shortest version which led you to be greedy to forget about a person you once loved. You ended up trying to find her in every girl you've met, because you yearned for her presence and warmth. Although, it only led to a rabbit hole that you couldn't get out of, you stayed with her roughly for a few years until you went on separate paths.
I want you to know that I'm sorry for the way I've handled things in the past, I wasn't being a good friend or classmate to you back then. It would've been avoided if only I was being careful enough with the words I've spoken, with the warnings I've said. I know what I said about you before created a scar that can never be undone, and an apology isn't enough for you to cater forgiveness. Don't worry, I don't need it. I just wanted to apologize for my actions 6 years ago and I hope that you continue to be well and happy ^_^
I'll always be proud of what you've achieved even though now you might have abandoned your dream on becoming a mathematician. You'll always be a good mathematics tutor to me, even though I don't understand half of the things you've taught me. I am grateful however, for the answers you've shared with me during activities that I needed your help with. Thank you for being such a good friend even though I wasn't.