Matt (assuming it's Matt),
The only thing not totally clear here is the staging of the disease (but you do say "in the early stages"
. Late-stage metastatic (Stage IV) is a different animal than the previous stages. It sounds like she's at an earlier stage. Regardless, I'm sure she's scared and getting a good look at her mortality.
You don't say how old your wife is, although you mention that you're in your late 50s yourself, and I know endometrial cancer is more likely to hit women who have gone through menopause. If she young enough and hasn't, however, she may also be mourning the loss of her child-bearing years and/or even the loss of her female identity to a degree (depends on her personality).
Unfortunately, in medicine, not everything can be "sorted out" and in cancer in particular, there can be many curve balls. I would like to think that if no one is talking chemo or radiation, and only surgery, if it is really an early stage, that you're likely to have a positive outcome, assuming she's in decent enough physical shape to withstand surgery.
(IN MY OPINION) As to what you say or don't say, take your cues from her to start with. Don't trivialize it, but be what I like to call "cautiously optimistic". If she's worried about minor things while she's down, such as, who will pay the bills (if she handles them), who will keep the house cleaned, what if she loses her job, reassure here that those are minor things in big scheme of things, and she needn't worry about those - you'll take care of (or arrange for) those - her focus should be getting better. People are behind her, people are rooting for her, she should do what she needs to do to get health and not worry about the rest. As someone with cancer myself I find those the most reassuring - don't surgarcoat, recognize that you can't magically wave a wand and make it go away, but you CAN take everything off the plate for her so she can just focus on getting better. Marshall your forces with family and neighbors, employers (if applicable), and move the furniture out of the way so that she can just focus on what she needs to do without stressing about the rest.
On edit: If you want to PM me (outside of this forum) please feel free to do so, if you think I can offer any help/support.