I think you need to do what makes you comfortable and what you need to do.
I had to mix feed with my DD and I felt bad to begin with but then I thought about it in the way that I need to do what I need to dofor our beautiful baby girl.
Dont beat yourself up about this - there is nothing wrong with moving on from BF. You might even find that you are more comfortable and relaxed when you move to formula as you will have more time to spend with your baby girl.
Keep your chin up x