I recently asked my best friend from college to be my maid of honor. Backstory on this, I was planning on asking her and if she said no I have three sisters and I was going to then make one of them my MOH and the other two my matrons of honor so that the other two sisters wouldn't be disappointed. I had thought too that my other two sisters would definitely be upset if I chose the third as my MOH.
My best friend said yes which was a surprise since she lives on the other side of the country. So, I told my one sister who was going to be my back up MOH option and she reminded me that I had actually promised her last year that she could be my MOH (before I got engaged). I knew we had had conversations but I couldn't remember the details which is why I went with the plan above. I definitely messed up by forgetting this big important detail before asking my best friend. Now I feel terrible because my best friend already told me she didn't think she would be anyone's MOH except her sister's and I want to keep my promise to my sister. Am I a terrible friend to go back and ask my best friend if I can keep the promise I made to my sister? Looking for some advice on how to fix this 
