Hi Joanna,
How old is Tess? Are you feeding per the I instructions on the food packaging? How often do you give treats? Have you tried feeding her a weight management food?
What you can do to get some weight off her is try the green bean diet. You substitute half green beans for her regular kibble. You can use fresh, low sodium canned (which I use) or frozen. Many of us have done this for years to get a couple if pounds off.
Aside from food, you might want to get her thyroid tested.