Just go with it. Take the pressure off yourself. Your DD won't love your MIL more; she'll adore her as a granny, but you're The Mummy. And you always will be.
I had similar worries, my DCs have two MIL days. They adore her (and so do I). But I am their mummy and their bond with me is unaffected by their trust in their grandma and their love for her. I'm number one . You are with your DD.
Take the long view. If there is a day here and there where she seems to gravitate more towards your MIL just flow with it, don't react. In the long run there will be absolutely no doubt who the most important woman in your DD's life is.
Good luck returning to work, it take a little time to get back into the swing but it's ok sooner than you know it.