you need to understand that talking negatively about his current relationship partner isn't going to make him fall in love with you. It's just going to show everyone who cares about you that you're willing to cross boundaries in order to in his life. Your letters all have the same theme: "She's turned you into someone I can't openly love." Their relationship isn't anyone's business but theirs. I realize unsent letters isn't indicative of real world behaviors, so I'm hoping that this internalized toxic entitlement toward their relationship is only in venting and not from an realistic place of entitlement. It doesn't matter how much you love him or miss him.
He isn't going to see your beautiful letters and dump his girlfriend. He isn't going to wake up to how much better you are than her, and run to you. People date the people they want to date. Anything after that is just diplomacy to spare feelings.
You are beautiful and lovable. You don't need to keep begging someone to validate you by trashing their girlfriend. If she's toxic, it's not your business. You can't save him from a relationship he wants. You deserve someone who shows up on your doorstep without using you a triangulation tool.