The only thing it will change is how you feel. I know it will make you feel terrible. Sure you're tired and things are tough, but adding guilt to that is not going to help. You are doing an amazing job love. The first years of a babies life i have found tough, but as you know it doesn't last forever. Soon M will be sleeping like a champ and you will hardly be able to remember this time.

M is getting all she needs from your amazing BM. Sure giving her formula may make her sleep a little longer, but only because it is more difficult to digest and will set in her stomach for longer. But i'm not sure sure that is a good thing.....

On the other hand, if you WANT to give her formula and feel it is the right thing for your family, then do it. Just be happy about whatever decision you make.