You didn't get to choose your parents—not intentionally anyway. Perhaps in some cosmic sense, your spirit/soul/whatever-you-want-to-call-it chose your dad's and my whatever-you-want-to-call-it. But, in this concrete world, in this permutation of our existence, you didn't choose us; I didn't choose my parents and your dad didn't choose his.
Here's the thing about parents: they(we) will disappoint you. I will disappoint you, and you need to know that those moments will suck for both of us. You also need to know that I promise you, I will always try my hardest to make it better. If I hurt your feelings or if I let you down, let me know if I don't know already, and I will do everything in my power to make things right between us.
There will come a time in your life when you will get to choose whether you want to be in relationship with your parents. I hope when that time comes, we won't have given you any reason to even think about not being in our lives. But I know that sometimes (grown) children have to make the difficult decision to end communication and/or relationship with their parent(s), and I can understand how someone would come to that conclusion.
Also please know that I will not choose to end my relationship with you. I will always try to be there for you and support you in the ways that I can, and in the ways you find most helpful. (So please let me know when I'm not doing so well at it!)
In the same way, I will let you know (when it is appropriate) if you ever hurt my feelings or disappoint me. It's only fair that someday our relationship will be much more two-way, although I will always be your Momma. (As an aside, I don't want you to worry about disappointing me with the choices you make. As long as you're safe and happy, I may not understand your decisions, but I will be happy for you and I will support you in making your own decisions.)
I will disappoint you, but I hope you will forgive me. I love you tons.