This is one of my utmost favorite names. As a nickname, it's very good. I do prefer the nickname Tessie over simply Theresa. I also like Tessie for names like Tessa or Contessa. But I especially love it as a full name on its own, which I'll admit is a bit saccharine and probably wouldn't "age well" in some folks' opinion. It's simply so pretty of a name that I don't care, and if anything, I think Tessie on an adult is refreshingly sweet.
Personally, I think Tessie is a darling name. I have a black and white cat named Quintessa, and I call her Tessie for short. Tessie sound soft and sweet but not spineless. It sounds youthful but not overly childish. It's great for an animal or a human!
But I especially love it as a full name on its own, which I'll admit is a bit saccharine and probably wouldn't "age well" in some folks' opinion. It's simply so pretty of a name that I don't care, and if anything, I think Tessie on an adult is refreshingly sweet.