Having a big/long face isn't always a bad thing. My draft has a big head, but it's nicely shaped and he's got nice features so I don't consider it a flaw.
It's when the horse's head isn't attractive that it's considered a jug head. TWH's are supposed to have large heads, but that doesn't mean they're jug headed imo. A lot of them have very nice faces. Your mare might grow out of it, and she might not. You'll have to wait and see.