If the therapy has already worked, then it means you have real ways to improve, and that alone is amazing. Relapsing happens, and it is not your fault. Many people do not realize how much effort it takes every day to stick to a method. There will be moments in your life when you feel low, less motivated, and the stutter becomes more noticeable again. That is okay. I think your father was probably clumsy when he told you that you will regret it. I know this because I have a father who is a bit like that too, and who loves me deeply no matter what difficulties I go through. I was older than you when I finally managed to express things to him. Your father probably believes that speaking fluently is what will make you the happiest, but he may not fully understand what you have to endure every day to get there. The best thing would be to try to tell him all of this, to explain that it is not just shyness. You can tell him the same way you wrote it here, or even write him a letter. He will still love you just as much and will better understand what you are going through. It could really unlock the situation and allow you to be supported. You should not go through this alone.