I want to start this letter by apologizing for my skepticism. I know that my attitude has been less than supportive and I regret that I have caused you any distress.
I know that you have been working hard on this project and I want to assure you that I now understand why you have been so passionate about it. I should have taken the time to listen to your ideas and to understand your motivations.
I realize now that my skepticism was a result of my own insecurities and I am sorry for letting my own doubts and fears get in the way of our relationship. I should have been more open-minded and supportive of your efforts.
I understand that my skepticism has been a source of frustration for you and I want to make it up to you. I am committed to being more positive and supportive of your efforts in the future. I promise to listen more and to offer constructive feedback when necessary.
Once again, I apologize for my skepticism and I hope that you can forgive me. I value our relationship and I want to make sure that we are both on the same page when it comes to this project.