The answer is that you will not always be with Millie...plain & simple. It's too easy to say I will always be there to keep our dog from harms way but, that just isn't realistic thinking. You won't.
It only takes a second or two and then you've defeated your whole theory of you will always be there. Then...you'll have a whole new bag of tricks to contend with...and it's a burden for Millie and yourself to have to go through.
Be safe for Millie & yourself and spay.